A line filter or voltage filter is usually used to help remove electrical noise and feedback into the truck. I have never had to use one on a 12 volt system usually only on electric forklifts for rf terminals or other accessories. Maybe it is what you need? Strange that worked fine for 2 years, maybe a diode has gone bad somewhere. Sorry i could not be of more help to you. Does the truck have a static strap if so maybe it isn't conducting to the floor anymore?
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