Like Rickie said motor first. Here is what I would try first. Pull cowl, trace down every wire you can find that goes to ground, remove those and clean area good along with ring eye. Make sure each one has a star washer that bites into block. Then go to wiring that is attached to battery and check each ground wire to make sure wire and connector are connected good. Check that power wire to finder is not wrapped around or laying side by side with main wires to motor to tach.