Testing neutral safety switch

Disconnect it from the starter relay. Connect a multimeter lead to the NSS wire and the other lead to the case of the tranny. Should measure <10 ohm in P or N. Then, measure to BATT-. Should be about the same. If not, you have a bad "block ground". It is below the tranny fluid level so you can remove it w/o worry.

If tests above fail, remove it and test it alone, pushing the plunger in with your finger (and moving other lead to NSS body). If that works, the rooster comb in the tranny is likely the problem.