The tach inside the car on the dash, yes.
It's not so much that the DITC can go bad, but that corrosion on the connectors can keep it from working. Or the sensor itself can go bad... in which case you're F'ed, time to convert to EFI, as the sensors are no longer available... unless you can find a good used one. But they're rare, since the cars are rare.
I think you should read my post here before you go any further:
924Board.org :: View topic - 931 Resuscitation - how to resurrect a long-dead turbo
Certainly, you can focus on the electrical/ignition stuff, before spending any more money. But you should read it all.