No, the 3M stuff will not effect the GPS signal at all.

 

I am working on a similar installation. I hardwired the female end of a cigarette lighter in already. I ordered the ball attachment adapter for a Garmin Nuvi. My question is, what is the most unobtrusive way to cut through the cubby to thread a wire through. I am going to thread in a mini usb cable to attach to the Garmin cable. Do I have to go through the whole process of removing the radio assembly, or is there a way to drill or cut a small enough hole, the thread the wires through the hole so they come out at a point where I can reach and work with them? Is there enough open space behind the cubby to thread a wire or pull one through? Thanks!

 

Just remove the radio, the ease of the rest of the installation and reducing in chance of damage is well worth it.