Your post isn't very clear: does this happen during testing (with the PC outside the car?) or only while it's in the car? Because if it's only while the PC's in the car, check your grounding. Search to find out what to do.
If it happens during testing, make sure you have a real case and it's hooked up right to make sure it isn't grounding. If it still happens, try putting a ferrite bead on the speaker wire as close to the amp (or head unit) as possible. You can buy them at radio shack for less than a dollar.