First of all welcome to the Pico forum
I am not seeing an attachment on my screen can you check at your end please.
So that the very knowledgeable people here can offer some opinions it would be good if you could post what will be the likely questions.
What year is the car?
How many wires does the alternator have?
Can you post a wiring diagram of the alternator circuit?
Anything electrical added / removed / modified?
What testing have you done, especially volt drop/load testing of powers and grounds?
Are the replacement alternators you have tried new / used / re-furbed?
Non standard cars are always a bit of a nightmare when you cannot see them, so the more info the better.