Tachometer replacement

Hei

Just got hold of a combined tachometer/ clock to replace the BIG clock in my "new" MB 230 CE. The instrument look nice, both original VDO but so to the big question:
How do I connect it up??? Do I take the signal from the coil? assume so, but where does the other wire go??
It has the original transistorized ignition system and I don't want to damage anything.

Any suggestions appreciated......

Frithjof (Norway)
MB 230 CE - W123, 1983 model.

[mbdoc]

The tach needs a key-on power supply & trigger signal fron the ignition module. The trigger needs to be connected to green/yellow wire at the diagnostic connector. POSSIBLY the factory wiring harness has this 2 pin connector already there!!