Radio Shack makes a filter for about $20. i tried it in my old altima and it killed all speaker wine. I put the filter in the positive lead going into the head unit, then the filter has its own ground that i screwed into the frame.
your best off not using any filters if possible, but if you do decide to get one make sure u get a power filter and not just an RCA filter (did not work tried 2 diffrent ones).
