Yup, definitly a ground problem. The empeg's case is connected to the ground of the player, so when you connect the wire to the frame, you're grounding it. It could be that the ground wire has come loose on the sled, or that your ground wire in your car is bad. Check the sled and see if that looks like the problem, or use a volt meter to see if your ground wire is good... Or just ground the sled and forget about it...

Matthew