Faulty bulbs, ballasts or ignitors? I had my driverside light go on my 370 a few months back. I actually replaced bulb, ballast and ignitor on mine as I had ordered all 3 parts just in case.
But you could try swapping these 3 items between the 2 lights to see what the issue is? Change bulbs over first and try the lights, then switch ballasts and finally ignitors etc.
Worse case get new sets of everything for both lights and do as I did?