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1988 mazda b2200 w/ issues >.<

So my truck is having issues.
Started with an installed tach. Tach shorted the coil. Replaced the coil, new coil blew. Replaced again, coil blew again. Tach is not hooked up, and hasn’t been.
I know its the coil because: if I use a spare wire to hook positive side of coil to the battery, it starts. I think its negative side, but I can’t remember (at work).

Wtf is the problem?! I’m just so confused. I’ve been told alternator, voltage regulator, etc but all that comes before the battery in the circuit and the battery is being charged just fine.