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Positive ground = Ignition switch to (-) on coil, (+) on coil to distributor. Negative ground = Ignition switch to (+) on coil, (-) on coil to distributor. That being said, it is unlikely the polarity being reversed is burning out the coil. Having the polarity correct simply makes for a hotter effective spark. Coils do NOT (with very few exceptions) have on "internal resistor"). A coil truly designed for 12-Volt use simply has more primary windings to function with 12-Volts. Bad spark plug wires, including the coil wire, can cause coil failure. A bad condensor MAY have some effect on the coil. Also, the coil will run hotter if the point gap is less than "spec". Also, have you checked the charging system voltage to be sure the 12-Volt system is not overcharging? Battery voltage readings much over 14.5 Volts, or so, with the engine revved up a bit indicate overcharging. What is the make and part number of the coils that have been failing?
Lots of folks are having good luck with the NAPA part # IC14SB, which is a TRUE 12-Volt coil.
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