Well, I guess some of the pink tach wires had a fuse and some didn't. But after all these years you might not have the original wire anyway.
On the plastic circuit, tell ames you need the early 78 circuit (if you have the round tach plastic housing).
On the tach, yeah, it's possible you've got a back coil lead. Try the light bulb thing to see if your getting a signal. I'm not sure what a volt meter would tell you. The service manual doesn't suggest using one.....at least not that I could find.
If you haven't done a tune up, you could pull the cap and rotor and change them. And, also pop in a new coil and module. I do that every ten years or so to avoid getting stuck. The cap and rotor on my daughter's car gets replaced every two years....just before I get the car smogged. The coil and module ten years is about right unless you put lots of miles on the car.
The other thing to do is to trace that tach wire from the distributor to the tach just to make sure there aren't any breaks. Also, if you pull that connector off the tach.....don't break the ears....I hate that!!! When I see those connectors at the salvage yard I cut them off and bring them home.