Tirerack.com
Check out the SVT Mustang Racing Rims in Black 18x9 $275 / ea
Here's a Link to the upgrade garage
http://www.tirerack.com/upgrade_garage/WheelSearch.jsp?&autoMake=Ford&autoModel=Mustang+GT&autoYear=2000&autoModClar=
I had the same problem with my 97 Contour. It was the ground wire if I remember correctly. But I had an aftermarket system, I don't know if this holds true for a Stock system.