It's been a lot of time since I last got into this site.
I'm willing to share a very easy mount for Garmin Nüvi GPS:
(https://4.bp.blogspot.com/-W80wwAMhHLQ/VtHvG6xKtKI/AAAAAAAADK0/_CdPYFz-4t4/s320/GS500_garmin_17mm_4.jpg)
I needed a ball that could be welded to the Steel bolt grade 6.6 in the steering stem.
I purchased a 17mm stainless stell grade 304 ball from http://www.bearingoptions.co.uk. Using a ball bearing ball is a bad idea because they are hardened and they will not weld easily.
Once I got it welded, I went for the power supply:
(https://3.bp.blogspot.com/-fqYreeK8k6E/VtHvGfEKooI/AAAAAAAADKs/6_mlXYfKMjA/s320/GS500_garmin_17mm_1.jpg)
Ebay is your friend: a 12V to 5V 2A converter is easy to find. I glued it inside the headlight housing using duble side tape.
(https://4.bp.blogspot.com/-jrrFiMrZgVs/VtHvGeVqRII/AAAAAAAADLA/sLQADqcD9E0/s320/GS500_garmin_17mm_2.jpg)
I got the power from the fastons of the side lamp holder: Brown is +12V white-black is GND.
Then I placed the waterproof USB connector between the handlebar and the front brake pump with a little drop of glue.
(https://4.bp.blogspot.com/-vElsthiMeUw/VtHvGfId4YI/AAAAAAAADLA/RA_yqn4rsFE/s320/GS500_garmin_17mm_3.jpg)
Got it :-D
(https://4.bp.blogspot.com/-YU79Bm2xWs4/VtHvG0XYoPI/AAAAAAAADLA/5KDqdm14R30/s640/GS500_garmin_17mm_5.jpg)