I like the idea of the built-in gps, but it would have to have an external antenna option. A single wire to run for the external antenna would have made things much easier (for me) than dealing with the serial connector and short extension along with the 12-5vdc converter.
As far as interference goes, I had a 2011 Dodge Ram, the gps-18 on the dash, would work fine sitting in the driveway, but it would go into "searching for sats" as soon as I started driving. I tried moving it to the top of the windshield, still no success. It HAD to be outside to keep a solid signal. I also tried a different gps(Magellan sport track), you could see the sat signals drop off as soon as I started the truck.
I had similar problems in a couple of cars I've had. I tried using the gps on the rear deck, while it held the satellites pretty good they would go into "searching" randomly, sometimes for miles. I suspect the problem was some metallic film in the window tint. The solution was the same though, had to move the puck out to the trunk lid.
Current truck, '14 Dodge Ram, no problems with gps-18 on the dash. Still would like to have a one-wire antenna out on the roof. Hmmm, maybe another winter project for me to look into.
Jeff.