Working on one of those projects now. There was a post on this forum a while back where someone basically gave the detailed instructions on how to install a tachometer. Might try a search for "tach" and "tachometer". Also, check out this thread:

adding a tach to a 240D (standard tranny)

Besides the tachometer, you need the pick-up sensor, located under the vacuum pump above the crankshaft pulley, the sensor mounting hardware, the tachometer signal amplifier located on the left fenderwell (a back cylinder), and all the connectors. The sensor plugs into the signal amplifer connector (after removing the two screws holding the connector on the bracket).

One wire to the tach is a hot wire when the ignition is on, the other goes to the signal amplifier. On the signal amplifer, one wire is hot when the ignition is on, one is a ground, and one goes to the afore mentioned tachometer. The sensor is a plug that plugs into the amplifier. From what I found out, that's pretty much it. Sorry, don't have the colors of the wires or diagram right now, but the tachometer is labeled for two of the wires. The diagram from any 300 turbo should have the wiring info. Plus if you get the connectors, you can match the colors of the amplifer to tachometer wire pretty easily.

One fly in the ointment is the sensor. The crankshaft pulley damper needs to have a pin sticking out that the sensor senses as the pulley rotates. If this is not there, the sensor will not work, then the damper would need to be changed. I don't think the 300 damper is the same either. You will need to check your engine. I believe some already have the mount for the sensor as well.

Good luck.