I believe the tach gets its signal from the PCM. I'd check that there's not moisture buildup on the connectors at that end, and if not there, you'll have to work your way up the harness. I had a similar problem a couple of times in really wet weather with my 95, and found that the sealing at the connector was not very good. The connections are different in a 96, I think, but on the older ones, it just happens that the tach pin is near the bottom, where it's likeliest to get wet.
I think the 96 has the same layout as the later models, but I can't find a specific pin for tach in the 99 factory service manual, whose wiring diagrams are a shambles, difficult to read and lacking a good deal of information altogether. I suppose no information is better than wrong information, which is present in both the 95 and 96 FSM's (Hint to 96 owners: if you ever replace your radio, check the wiring with a meter, don't trust the FSM pinouts!).
