[tommyduncan]

Good info but not just a GMC thing. Anything with points has some form of resistor to accomplish this. Some makes used a little off white rectangular looking one that usually screwed to the firewall.
There is another wire that goes from the starter to the negative side of the coil also. This is to supply the ignition with a full 12+ volts during startup. It only works when the key is turned to the start position. This wire is often removed because people assume it does nothing.