If you're using a Nuvi on your bike, I have an additional recommendation. The USB power port on the Nuvi is not a good secure fit with the USB cable, even when brand new, and the cable can vibrate around in the GPS unit when you're riding. The power port is a point of failure for these auto-based GPS units when you use them on a bike. I found a simple way to secure the cable in the GPS power port that keeps it secure and less susceptible to vibration. I've been using the same Nuvi 2455 on my bike for at least four years now and many thousands of miles with no issues.
My Nuvi was in the typical RAM cradle. To secure the USB cable in the unit, I took a small piece of aluminum angle (from the hardware store) and glued a small piece of rubber to it. The then positioned the angle with the rubber pad up against the power cable so it just barely made contact. I then bolted the angle in place to the RAM cradle. Then I used a piece of Velcro tape to secure the USB cable to the aluminum angle and the power port on the Garmin. This secures the cable so it wont vibrate, and it keeps the cable firmly mounted in the power port so there isn't any strain on it. The rubber pad helps to absorb some of the vibration.
I had the same issue you did with the OEM 12 volt outlet and a standard 12 volt plug not staying secure. After a frustrating trip to Missouri and back where the cable kept disconnecting, I decided to dispense with that type of power connection for the Nuvi and just switched to a buck converter and a USB outlet. I put a couple USB outlets on a homemade shelf (made from an ammo can) above the instrument pod, and I plug the GPS in there. The power cable screws into the USB outlet, so the cable is secured at both ends and can't vibrate loose.
