Ground the amp in the boot with a suitable (thick) cable. Don't run it directly to the sled.
Then run a wire from the empeg to the grounding point of the amp (not to the amp itself, to the grounding point, very important). That wire does not have to be thick. 0.75mm² will do fine.
Don't ground the amp directly at the battery.
If you still have ground loops, sometimes it helps to isolate the amplifier's case from the body of the car (by mounting it on a piece of MDF or so).

Frank