Follow the black wire. Mine was a small (maybe 1 foot?) wire that went from the negative terminal to the firewall. I forget if there was another ground somewhere. I cleaned the crapola out of mine because bad grounds are notorious for creating electrical gremlins. Do you have a volt meter? Can you test your battery, or take it to your local parts place to have them test it?
I used one of these, but a wire brush or sand paper will work just fine:
My ground connector that comes off negative terminal (black wire):
Between battery / battery box and passenger firewall:
I didn't want the exposed metal to rust, so quick paint prep:
All I has was green rustolium:
A little love for the terminal:
A blurry pic of the cleaned up terminal: