I have the exact same problem with my XJ, but mine is intermittent and is on the left side just like yours. It usually only does it if I've been driving in the rain and then once it dries out it acts fine again. I'm guessing I have a short in the front somewhere that is causing it when wet. It could be a couple wires that have rubbed the insulation off of them. Let yours sit for a day with the sun out or take it for a drive and see if yours does the same thing.