Bentley shows the red wire going from the overdrive switch to the overdrive relay. The red wire is what applies the ground to close the overdrive relay through the transmission over drive switch. Shows it on the transmission in the drawing.

I do not have a manual tranny but in looking at the diagram I do believe I see a way to test to see if the unit will engage. Get a decent switch and some wire long enough to plug into the relay contacts and some spade lugs to plug into the relay connector. You can try the redneck overdrive relay.

Pull out the overdrive relay and plug one side of your switch into 15 socket (Blue) and the other side to pin 87 (yellow). You want the switch open when you start the test. Get the car up to speed and when ready flip the switch to closed. That is going to put 12 volts on the solenoid so If the overdrive kicks in you have a working OD unit and are headed to the shifter boot or the relay.

Well the black wire on the solenoid should also go to chassis ground. You will want to make sure it does have a good solid ground if it does not engage. If you have a ground and putting 12 volts on the solenoid does not engage the OD, slide under with the switch closed and see if you have 12 volts on the yellow wire at the solenoid.

Not the cleanest trouble shooting ever but at least you will know if the unit will engage or not.

I have an Auto 86 Auto Tranny model and you will find a lot of bare wires under there on the fuel pump wires and other places. Move the wires along the transmission and the shifter and make sure you don't have 86 crap insulation issues going on.

Good Luck,

Paul