Get out the voltmeter
I would do a bit more troubleshooting before concluding the diodes are at fault.
First check for voltage at each end of the "left/right headlight motor" fuses (my Bertone wiring diagrams refer to these as fuses 9 or K and 10 or I). There should be battery voltage at all times at both ends of each fuse regardless of ignition switch and light switch.
Then check the voltage at each end of the "headlight closing relay" fuse (my Bertone wiring diagrams refer to this as fuse 11 or O) with ignition on and light switch off. There should be battery voltage.
Next check the voltage at each end of the "headlight opening relay" fuse (my Bertone wiring diagrams refer to this as fuse 12 or H) with ignition on and light switch on. There should be battery voltage.
Then measure the "sense" voltage from the right headlight motor as you manually turn the right headlight through an entire up / down cycle. My Bertone wiring diagrams show this wire as GRY/BLK. Ignition switch on, light switch on. You should see battery voltage during half the cycle, no voltage during the other half of the cycle. You should also hear the relay click somewhere around headlight full up and full down.
Repeat the above with ignition on, light switch off. You should see battery voltage at one half of the up / down cycle (opposite cycle from above).
Repeat the last two tests for the left headlight, the sense wire here also shows as GRY/BLK in my wiring diagrams.
With the results of these measurements we should be able to make a very good guess at what is wrong.
Caveat: I don't know how well my map (Bertone wiring diagrams) matches your terrain ('79?)
