You could have a poor feed to the starter solinoid caused by bad ign sw, NSS, or K38 relay circuits.
The easy test point for diagnosis is at the plug to the left of the Master cylinder fluid reservoir, on the engine inside firewall. Unplug this 3 wire connector and run a jumper wire from the bat + post to the center terminal on the firewall plug [ not the harness you just unplugged]
Be sure the car is in Park.. This jumper brings direct 12v to sol, which should energize the starter and verify circuit
..if NO starter engagement, then sol/starter/or wires to it are suspect.
This is just a test point in the circuit that we use to seperate the relays and switches of the start circuit in the cabin from the actual feed in the engine compartment..
If that makes the starter spin, your problem is upsteam in one of the relay/switches I mention in the cabin..if not , then you know the problem is downstrean from that point.