Alt alternator lights up when I place in reverse gear
 

It's an 89 model 3.2. In daily use. Alternator was replaced about 15k miles ago.
Place in reverse gear and the alternator light comes on!
I'll take it to an autoelectrian but I'm dumbfounded on what it could be. Any clues?

How about a shorted wire or a bad backup light switch.

I just researched this on my '85. The ALT light connects between alt and ignition switch via a diode. So the light goes on when alt is below battery by 0.6V during operation.

The connection to alt is via 3 diodes to the coils. So it's monitoring the AC output inside the alt, not the alt output. I could see this in schematic.

This does not answer your question but may help in testing the voltages on each wire.