[targa80]

I have looked at the 81 schematic and the connections from ignition switch terminal (50) yellow wire connects to connector T1-E (on luggage compartment floor) to connector T14-1 (on regulator panel in engine bay) to terminal 50 yellow wire on starter motor solenoid. You have several options:
1. Disconnect battery and perform an ohm continuity test from disconnected ignition switch terminal 50 to disconnected starter solenoid terminal 50. This test would require a long jumper wire with alligator leads. The result should be a very low resistance reading. If the resistance reads infinity move one lead to the closest connector and retest until you find the open contact. Also, test wire to chassis ground for a possible short to ground.
2. Disconnect terminal 50 from starter solenoid and check for voltage on wire with ignition switch in start position. No voltage move to next connector in circuit and check again.

Good luck with your troubleshooting.