You can't just hook up most tachs because of the ignition system, and there's no independent tach signal. It has to do with the coil being split into two parts; using a tach on the 4-cylinder setting will cause it to only read 1/2 of your RPMs. The only tach that doesn't need an adapter is one that can read 2-cylinder engines.

 

Thanks hemi4spd, I'm not sure why it worked on the '97 but doesn't matter now. I'll pick up a signal converter for about $75.

 

...and darthroush, I did not know that about the 2 cylinder engine tach...thanks