On my 560SL, there are 2, both at the top of the cluster. One is 'hard-wired' to the cluster (gray or red, I forget which, one is attached to the holder) and gets its power (or ground) from contact with the casing when you turn/insert it into the socket. The other has spring-loaded tangs that contact the printed circuit when the holder is pushed in and turned.
Both bulb holders are about 2 1/2" long black plastic 'tubes' that put the bulbs at the front of the cluster. One illuminates the tach, the other all the other instruments.
I'll add that they are both really flakey, in that I tested the bulbs for function, inserted them properly, then made sure the holders were seated right. I even verified that they were working with the cluster out of the dash, then again after putting everything back in. After re-installing the steering wheel, I STILL have to tap the cluster from time to time to get 'em both to work.
And, BTW, solder a wire across the rheostat terminals while the cluster is out. This will eliminate this pesky piece of s**t as a source of trouble (like your lights go out while driving at night!)

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1986 560SL
2002 Toyota Camry
1993 Lexus