Charger of Nuvi 1300 works with 3790 (No Traffic) but ok for testing.
Another test, Bring the powered mount inside and connect usb cable from computer to Mount (Portable Hard Drive Cable will work).
Process of elimination.
It the problem the Nuvi? Is it the charger ? Is it the mount ? Is it the car ?
Have you cleaned the contacts between the mount and the Nuvi both sides ?