Sounds like a bad diode in the alternator. When one of the six is shorted or open, you will lose very nearly two thirds of the alternator's rated output. That means you will be lucky to get 30 amps from a 90 amp alternator. A pair of headlights, not including all of the tail lights and dash lights takes over ten amps. Another ten goes to run the fuel pump and engine. Ten more amps to run the heater fan on a medium speed, and that 's all the defective alternator can handle.
Have a load test performed on the charging system. Find a shop that has a tester that can measure AC ripple. That will be very high when one diode has failed.