Would a starting cable be any good?

You mean like a jumper cable? Gosh, no, that's overkill.

Or buy another amp power cable and use that for ground instead of 12V?

Right, that's the sort of thing I'm talking about. But don't do a permanent "serious" cable run at first. Just wire it up "loose" and see if the noise problem goes away. If it doesn't, then start looking elsewhere. If you can do the experiment without spending any extra cash, that's the best way to go.

Another thing to check: Make sure none of the cables the phone-installer-guy put in are grounding against the chassis anywhere. For instance, they might be bent over a sharp edge of metal and their sleeves might be making contact with the car body somewhere. Check the entire cable runs for problems like that. There should be no contact with the body of the car by any component except at the exact ground points you've decided upon.

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?

No, what I mean is: Decide upon one single grounding point for all of your equipment. That grounding point is most likely a paint-scraped bolt somewhere on the car. Wire it so that the empeg and the amps ONLY ground to that one single point. Meaning the ground wire from the empeg and the ground wire from the amp both go to that single point, therefore you might need a long cable run for one or both of them. I don't want you "connecting the dash to the trunk", that would be silly.

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Tony Fabris