I'm asking this question on behalf of my brother.
My knowledge of Toyota trucks is limited so I'm will try to provide as much info as possible.

My brother is having a problem of not being able to start his truck.
Battery has been replaced
Starter has been replaced
Ignition Coil has been replaced
Ignition Switch has been replaced
Clutch Petal Switch check out OK with OHM and voltage
Start Relay checks out OK with OHM meter and voltage
All fuses in engine compartment, and kick panel have been checked, that are in a box.

The starter appears to be, not receiving electricity at terminal 50, the smaller plug-in wire.

Are there some other areas to check? Hidden fuses relays?

91 Pickup 4X4 2dr extended cab 22RE engine? I believe that is correct.

If you still check this form, I can dig out my service manual and look.

Connect a fairly heavy gauge wire to the starter solenoid + terminal (long enough to reach the battery).
Put the key in the run position .
Touch the wire to the battery + post.
If the starter cranks, it's the wiring betwen the switch and starter (fairly common in the 80's trucks).