I used to work at Best buy and had people coming in all the time with the same problem. If you go to the cell phone/PDA section, they sell an active converter made by belkin I think. I set this up for many customers for cell phones and PDA's, they work wonders, no problems, I'm sure it would work fine for the GPS. Just make sure if you buy it, to load the software before you plug it in, will save you a lot of hassels.
Matt