Assuming you're talking about a 93 400E like your sig line suggests -

Wherever they are there's 2 of them so you have to run the tach in 4 cyl mode.

Pull the front engine cover behind the left headlight and follow the cable from the middle of the distributor to the coil. I suggest to try that because in the S420 the coils are tucked just behind the left headlight.

Then of course - why?

Sixto
95 S420
87 300SDL