Heres some shielding tape that you could try spinedoc
http://www.stewmac.com/shop/Electronics ... _Tape.html
Of course skicrave knows this won't work but the fact is if you can figure out exactly where the interference is getting into the unit and you shield that area it'll solve the problem. The trick is wheres it getting in. I couldnt even guess without having the unit here and pulling it apart. Im not telling you to pull your unit apart but Id be pulling off my screen and seeing if I could put a shield between the screen and the PCB's that are in the unit behind it. Like I said before it might be the screen itself or no way to shield the area that needs to be shielded and then youre stuck.
...and dont blow anything up or short anything out! (if you do it)
skicrave,
Of course this is a common problem that many people are dealing with, and it would be a hard issue to correct. However spinedoc has said that they pulled their unit already 3 times to try and fix it. This is someone thats obviously bothered by it enough to really want to try and fix it. My suggestion is just that, a "suggestion". Im not promising results. It's a lot better then you're idea of just move your cable to the trunk. Proper shielding in the proper area of any device will inhibit interference. Depending on the car the unit is installed in and/or how its installed, it is "possible" that the case of the unit is not getting a good ground. Unless you've tried it yourself and failed I could do without your "it won't work" comments thanks.