Hi,
Could be an open ground trace on the PC Board that connects the RCAs to ground. This sometimes happens if the car Power Amplifier isn't well grounded and the RCA cables/HD Radio ground becomes a high current path to ground.
Sometimes, the pin for the ground shell breaks from flexing the connectors when the cable is plugged in.
USING ESD PROCEDURES, remove the cover, check the traces on the top and bottom of the board that go to the outside ring (ground shell) of the RCA connectors.
If the trace is open (burned), trim it, scrape the solder mask off 1/4 inch with a blade, and solder a wire across it.
Ross
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In SI, a little termination and attention to layout goes a long way. In EMC, without SI, you'll spend 80% of the effort on the last 3dB.