I'd check the fuses. There's one that feeds the instrument clock, cigar lighter and various stuff on my 300E. And these fuses drive me nuts as they develop bad contact in my car. I am sure theres something similar in your 420 too. Just check the fusebox lid for which fuse it is, and wiggle it around a bit and see if it helps.
I'm ready to replace my fusebox this weekend with ATO fuses.