In addition to Franc's suggestion above (very true, Franc. I had a neighbor who went through the same mind-numbing issue with his Benz years ago & was a wee bit P.O.'ed months later when another mechanic spotted the incorrectly installed side marker bulb. It was cocked to one side. Weird stuff!) I'd also check the negative terminal on your battery. Make sure it's tight & then follow the cable down to the frame & make sure it's tight, too.