Then run a wire from the empeg to the grounding point of the amp

By lengthening the groundwire coming from the empeg, you are increasing its resistance. This has the potential to induce a ground loop. I would only do this as a troubleshooting step, and only with a very heavy gauge ground wire.

Normally, you would ground the amp to the car frame in the trunk, and ground the empeg to a proper ground in the dash. In most cases, the ground wire on the car's wiring harness is plenty for this task. The only time you would need to do anything else is if you've already hooked it up this way and you're having to troubleshoot a noise problem. If I recall correctly, the initial post in this thread didn't say if he'd already tried it this way first.