I have a 1983 240D. I recently needed to replace the clock. I purchased a used one. The one they sent has the clock/tachometer combination. The clock works fine. I thought about sending it back, but then I thought that it might be really cool to have a tachometer in a car with a manual transmission.

What would it entail to "hook up" the tachometer?

I can't tell you how to adapt a tach/clock combination to your car.It would be a lot of work. You might have to change the harmonic balancer, because that is where the RPM sensor is located.
You can repair the present clock in your car fairly easy. Remove the clock and take off the rear cover. You will see two electrolitic capacitors. One is probably bad, But replace both of them. They cost about $1.00 each from Radio Shack. GooD luck
Bill Lewallen Lexington,Ky.