Intermittent Tail light '91 300E
 

Everytime I turn on lights the left tail light doesn't come on. If I tap the lens it illuminates. I've replaced the bulb but problem still exists. Any suggestions? Thanks

the rear tail light assembly is notorius for contacts loosening causing this type of problem. you might want to carefully inspect and clean out the socket in question. a little creativity with shims / springs might solve your problem. be careful with the bottom contact - it can easily snap off if you try to pull it up. the entire assembly is a $300 item so it's worth fixing...

I have also had luck with the packets of bulb grease, $.49 at auto zone. After cleaning/repairing.