Pyramid NS12 12 Amp InLine Noise Suppressor,
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This review is from: Pyramid NS12 12Amp In-Line Noise Suppressor (Electronics)I had interference on the radio when the 12v accessory socket was being used with car camera etc. Connected this suppressor in-line (blue + red wires) to the positive wire to the socket and to earth (black wire). Problem solved instantly.
One point - this is not a small suppressor at roughly 4" long by 1.5" diameter , so check you have enough room for it. Principally designed to go between the radio and battery but I knew my problem was with the 12v socket alone.