Uhm, one of them suggests to "connect everything to a common ground". How do you do that in a practical way, when your Empeg is in the dash and your amp is in the trunk?

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.

Of course, this defeats the "keep ground wire runs short" rule. You can try both (grounding to two points on the metal frame with short runs, or grounding to one point with long runs), and see which one works best. If there's no difference, then go on to the next diagnostic step.

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Tony Fabris