Workshop manual with a wiring diagram would be the first step. You more than likely have a wiring fault in the 3 wire harness from the engine to the dash. Just to give you a bit of a heads up, disconnect the 3 pin harness at the indicator light. The Blue/yel should have a constant 12v with ignition on. If not, go to the 3 pin harness on the engine side and see if the same wire has 12V. If not, check at the alternator,same wire, if not then check the same coming out of the main relay. I am guessing this wire has a break or corroded terminal somewhere since it feeds both the alternator and check eng light. I am assuming here the engine runs normally?