I have a 78 242.

I was at the pick your part - saw a 78 GT 242.
Many parts were gone - but can you believe it - the factory tach was still in the cluster.

On my 78 there is no wire in behind the cluster - like on my 87 245.

There are two locations on the ignition coil - one spade terminal has a wire that goes to the condensor on the distributor - the other spade terminal has a wire that heads off into the wiring harness - I suspect to the ignition switch.

QUESTION - which of these terminals is the one that I will connect to the tach?

Thanks in advance.

Every tach I've dealt with (not that many) has worked from the negative terminal.

Another thing you might want to check into is whether the coil on your 78 is "leaky". I had an old coil kill a nice new Sun tach once, and it turned out to be leakage, which gets worse as they get older. It didn't happen immediately, took a few weeks IIRC. And no, I can't remember anymore how I tested the coil. Looking at it in the dark with the engine running might do it.

if that tach works the same as your 87 it will connect to the negative coil terminal which is the one that goes to the dist.

The tach terminal will be the negative which is the one that goes to the distributor.