I would suggest you check the wiring leading from the alternator for bad connections or partially broken wires, particularly where the wiring flexes as
the engine moves, also if there is a connector in the wiring leading to the alternator. A simple quick way to get on idea if this is the problem is to
run the motor and wriggle the wiring whilst someone is sitting in the car checking the dash lights, if you can make the dash lights come on by wriggling
the wiring you will know you are onto something.