If it's like the 6 cyl HEI I have, with the coil mounted on top of the cap, that three-prong plug goes into a matching socket on the bottom of a rectangular extension of the coil housing. There are two blade connectors at the outboard end of that extension marked "TACH" and "BAT." Hook 12v to the BAT connector. If it doesn't have the integral coil, I dunno.
Joe
