Run a heavy-gauge ground wire between the trunk and the dash, and make sure that neither the Empeg nor the amplifier are grounding through their chassis.

Would a starting cable be any good?
Or buy another amp power cable and use that for ground instead of 12V?

I can think of ways to connect a heavy-gauge cable to the trunk, but to the dash? What kind of connector would one use?

Thanks for your help,
Pim