Tach Problems + Where's stock battery ground?

[Gnnr]

After trying to fix the last owners wiring stupidity, my car started to go haywire so I put the grounds on the battery terminal back to how they where, and everything started to work again. Except now my tach is messed up. The RPMs don't read at all (stays at '0') and my MPH goes up very slowly (in lazy fashion). I tested the fuses in the engine bay (but not under dash yet) and they're fine. What could have gotten fried? This is in a stock s14 with auto tranny.

Also, what is the stock location of the battery ground cable? The previous owner had some dumb crap going on and I don't think its even the right gauge size wire.

Thanks ahead.