I'm reviving this thread because I had the identical problem and found the solution.
The instruments and most of the lights in the cluster are all grounded from a common point. The ground comes in on terminal 1 of the three pin connector on the back of the speedometer. From there is goes along circuit board traces to a large metal plate on the top of the speedometer section where each side panel picks up the ground through a mechanical connection (hex screws through the two panels sandwich them together - not the best design but it works). The contact plate is on the top center and I highlighted it on the attached photo with a yellow "T".

If you are having problems with irratic gauge readings, especially when you turn on the turn signals or instrument lighting it is almost surely caused by a bad path for ground. In my case it was a trace on the circuit board on the back of the speedo section that had opened up. On the photo I circled the open trace in yellow at the top left corner of the board. I also circled the ground input (pin #1) too, for informational purposes.

I simply soldered in a jumper wire across this break and all was perfect again after that!

I am sure that a loose screws at the junctions can cause the intermittent jumpy gauges we've seen over the years on these cars too so cleaning them and tightly screwing them down will eliminate that too. Like I said, it is a poor design but can be fixed easily.

So, if you have this problem, grab an ohm meter and trace the ground from the point it originates at pin 1 to the plate that distributes it to the side sections and clean up those connections well too so there isn't any resistance (which is what probably caused the trace to pop on mine).

On the bright side I have gotten very good at removing the steering wheel and airbag - I can do the entire job and get the instrument cluster out in about 15 minutes.

Attached Thumbnails

 

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz