My pesky ground loop was reduced by heavier wiring to the battery.
It went away completely when I finally grounded the sled to the car chassis.

On your 'test' install (I'm picturing components and wires all over the floor of the car) use some alligator clips to ground various components and see if anything helps.

I'm not an electrical expert, but good luck!
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10101311 (20GB- backup empeg)
10101466 (2x60GB, Eutronix/GreenLights Blue) (Stolen!)