I had an issue once with a circuit running my newly installed Shurflo roof vent fan which made my TV picture scratchy. The manufacturer sent me a diode to put in the positive lead wire and that resolved my issue. They also told me I could reconfigure one of my 12V circuits and switch with a 'filtered' circuit. Apparently some of the 12V circuits are filtered while others are not. Since the diode installation resolved my problem I didn't switch circuits. I was told that the radio and TV booster circuits were filtered while light circuits etc are not. Might be worth looking into...