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...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"