My 300SD, with 425,000 miles has developed an interesting and growing instrument problem. First, my glow plug light would randomly light up for a few seconds, then go out. Always when warm and underway. Not a failed glow plug, and normal glow timing is fine. Cold starts are fine. Not the indication of failed plug during heating.

Then the seat belt warning light started to do the same. A few seconds on, then off again randomly. Yes, my seat belt was ALWAYS connected.

Now the water temp is doing a dance. It indicates correctly, then starts dancing above correct temperature but not all the way to the top.
Don't think is is purely vibration but might be road shock related. Live in the boonies.
Then it goes back to stable temp indication for minutes, then dances again.
They don't necessarily all error at the same time.
So I am guessing a failing ground connection! (or loose 13 pin plug).

I have the panel out now and all looks fine. The only brown I see going to the panel is part of the circular 13-or-so-pin plug, which looks clean.
Is there another ground for the IP?

I was thinking of splicing into that one brown and creating second, solid path to the chassis.
Any ideas are welcome, a real pain to put it all together and go for a test drive. I can't force the error but it happens on every drive.

As always, your wisdom in cherished and appreciated.

Thanks,
David