Its really not that hard to work out
The alternator only needs two wires and a ground to work.
Its truly as simple as doing the following:
With the engine running do all these tests with a multimeter on the voltage setting:
Get a good connection from the alternator case and put the - terminal on this.
The put the + terminal on the battery NEGATIVE terminal.
Make a note of the voltage. It should be LOW say around 0.2 volts.
Now put the + from the multimeter onto the nut connection on the alternator, and still keep the - connected to the alternator casing. Voltage should be AT LEAST 14v with the engine running.
Now get someone to bring the engine revs up to about 3000 rpm. Check the voltage again. It should be still above 14 but below 14.6 ish. If its outside of this range it implies a faulty regulator within the alternator. It may be at that at idle the voltage will be below 14 if you have only just started the car, but a quick rev to a couple thousand should bring it up.
Finally with the + still on the alternator + output put the - terminal on the + on the battery. This is where your most likely to see a voltage drop. The voltage here should be below 1.0v ideally much lower.
If this is high then easiest solution is to run a new thick cable from the + on the alternator directly to the battery. Although unusual to have to do this on these cars its not so uncommon to have to do this on old cars which often has underrated cable anyway.
Other than wear and tear, the other reason I have found alternators die quite quickly is from over heating. They typically draw air IN from the BACK and then it gets pushed out the sides. If the pipework is too close to the turbo down pipe it can kill them very quickly. This I have seen and have experienced 1st hand. The regulators will fail if they exceed about 100 deg very quickly.