Voltage Drop

You must have a short somewhere. With a voltmeter and the engine off, check the battery voltage with one lead disconnected and then check it with the lead replaced. There will be some voltage drop difference but a small one at that. Report back here.

I'd say bad body ground, based on heater blower not working. My 84 El Camino did that a lot, blower out, voltage dropping at traffic lights, dim lights. What I did was run a Ford starter cable from block to frame rail, and moved negative battery terminal from alternator bracket directly to engine block. Chevys use a pathetic jumper wire that goes from battery cable to fender to ground body. For the ultimate in grounding pleasure, run a jumper wire from frame rail to radiator support, as for alternator, be sure brackets are cleaned to bare steel where alternator touches bracket, a poorly grounded alternator will soon be a dead alternator.

Once you are properly grounded, test throwback voltage using a VOM right on battery terminals. Test voltage, start engine, voltage should climb with engine running..