I am in the process of doing similar for a Garmin unit. Like you I never ride in rain.

Also as the unit would only be required for long country trips and only infrequently around town, I wanted the GPS mounting to be easily removable.

For power, I considered using the front under-tank power port which required buying the BMW connector. However, in the end I have opted for a waterproof USB wired directly to the battery, and zip tied under the petrol tank to the front. No power-out for short stops, and the GPS will be unplugged for longer ones - one needs to remove GPS for theft prevention anyway.

I bought one similar to fused cable and outlet below, but opted for a single USB port as it has a smaller footprint - about AU$ 14 on eBay. I will probably fit the outlet onto the black plastics on the fuel tank front rather than handlebars to make it unobtrusive. Also, I can away retro fit this to the under-tank power outlet if I wanted the on-off option linked to ignition.

Yes, there are after market USB plugs that fit into the Hella (DIN) socket - about $30 here - I discounted these I didn't want the protrusion and you would still need to wire a waterproof USB cable through to the handlebars.

The RAM mount, also from eBay about AU$3, clamps anywhere along the handle bar and allows full tilt and swivel.

 

Thank you Anthony.

 

Anyone else have any input here?

 

A little update on my project to fit the Garmin.

The RAM mount works perfectly.

Problem is with the USB power port - when I connect the Garmin using both the Garmin and other USB cables, the GPS says the cable is incompatible and charging won't happen. I had no problem using these cables with my in-car USB charger.

Mr Google suggested:
1. Using a "Charge Only" USB cable (by effectively cutting the green and white the wires connected to pins 2 and 3) - this didn't work
2. The USB output may be insufficient - the USB port is rated 1 amp which should be enough - a multimeter indicated that the Garmin was drawing just .5 amp. Mr. Google says it should draw 1 amp to charge.
3. Making up a special cable - too hard!

So I am at a bit of a loss. The waterproof USB port may be faulty or simply underrated.

Has anybody else fitted a waterproof USB port and connected a Garmin to this?

 

Have you tried getting a usb cable from Garmin? I had a problem like that once and can't remember the details, but the cable wasn't of the appropriate internal anatomy. The proper cable remedies the sitch.

Just to jump in a bit, yes the car gps can certainly be used on a bike and if you slip it behind a windscreen for have some decent protection from the weather. Just carry a ziploc bag and a couple of rubber bands in case you get caught.

 

I've had success with the same or similar waterproof USB port purchased on E-bay (2 of them actually, one on my F800ST and one on my 2012 R1200R) with 2 Garmin automotive GPS units (250W and 2455LM). In both cases I hooked mine up to the dedicated power outlet using the BMW connector thingy. Instead of the RAM ball mount, I have one by Ztechnik that mounts to the left handlebar control unit resulting in, to my eye, a cleaner installation, though being a hard mount with no vibration isolator it likely transmits more vibration to GPS unit than the ball mount does - more on that below.

I can offer the following observations from my installations:

1) On power-up the GPS acts like it has been hooked up to a PC, but after 30 secs or so it will go into navigation mode and function normally. Apparently this is typical 'behaviour' for a Garmin GPS hooked up to a non-garmin power supply.

2) My installation goes through a lot of GPS cables. They'll work fine for a while but eventually will start going through the power-up cycle regularly, to the point where it never gets into navigation mode. Replacing the cable with a new one fixes the problem for a while, but it will eventually return. I suspect vibration causes the USB cable connector on the GPS side to wear resulting in intermittent power.

3) Be wary when popping the waterproof boot off. The tether on mine broke when I removed mine in cool weather.

4) From what I can make out the max charging current depends on whether the device is hooked up to a garmin or other power supply. For example, when turn on the service menu* for my 250W and connect it to a regular USB cable, it shows the charging current limit as 500 mA (and it seems to draw a max of about 150 mA even when the battery is low). If I hook it up to a garmin power supply the menu shows to the max charging current as 1000 mA, however I've never observed it to draw much more than 500 mA even when the battery is low. I should also note that notwithstanding item (2) above, even at the relatively low charge rate of 150 mA, I've never had issues running or charging the GPS on the bike's power supply though it definitely takes longer for it to charge than when using the garmin power supply. All that said, I have, on occasion, seen the unit show a max charge limit and rate on the bike as if it were hooked up to a garmin power supply, so go figure.
Also note that for my 2455LM, the charge current limit for a garmin power supply is 1300 mA, though again I've never seen it draw much more than 500 mA. The regular USB charge current limit and actual draw are 500 and approx 150 mA respectively.

* to access the 'service menu', press and hold the battery symbol in the upper right corner of the display for about 10 secs. Note that on my 250, this symbol is only visible when then the unit is not hooked up to a power supply. Once in the service menu, you can hook up a power supply and see the charge current limit and actual charge current.

5) I can't see it making any difference, but I did not include the supplied fuse on the USB cable seeing as the circuit is protected by canbus.

6) I had success with Dr. Strangelove's waterproofing method on my recent 'I've seen fire and I've seen rain' tour through Washington, Idaho, Oregon and Cali.

Hope this helps.

 

With the above information, I re-tried the USB port - I connect it with GPS battery near-flat. The spurious GPS message did come up, but the device went into navigation model, and continued working without interruption long term.

When I disconnected the GPS, its battery was still near flat. So, my conclusion is that the USB port provides power to navigate, but the Garmin software is preventing battery charging. This is no a problem as the device works!

I don't intend to leave the GPS nor the RAM mount on the bike - only for use on longer trips. It takes 15 secs to put on and off. I have put a small hole through the RAM thumb screw so I can cable tie both the GPS and the screw to avert a possibility of vibrating loose, and discourage loose fingers .

Wiring directly to the battery I made sure that there were no wires above the frame - I don't want an Erin style pyrotechnics!

BTW - no special Garmin cable is required

 

Glad to hear you have it working, even if the set-up only seems to provide 'maintenance current'.
That looks more like a 24xx or 25xx series than a 25x series, the latter of which is what I was basing most my observations on. It also looks different (newer) than my 2455.
Were you able to access a service/test menu to see how much, if any, current you are getting with your set up?
In my experience, it would take 2-3 hours to fully charge my 255 or 2455 from flat on the bike - much longer than with the garmin power supply. Neither of mine display the message you are getting but I'm pretty sure your unit is newer than either of mine so that might be a new feature.

 

...
That looks more like a 24xx or 25xx series ...

Were you able to access a service/test menu to see how much, if any, current you are getting with your set up?

1. My Garmin is a NUVI 2597 LTM

2. On current - I split a USB cable to measure amperage - the unit only drew a bit under .5 amp (of course voltage 5v from USB), none of which went to charge the GPS battery - when I unplugged after an hour the GPS battery immediately showed "insufficient battery"