Aloha,
Good to hear you are driving the old gal instead of those "Other cars"
A few possibilities for this:
If you initially had corrosion on the connection, this will cause resistance and, If the voltage drop under load across the resistance is great enough, let's say the voltage dropped to 6 volts during operation, this will cause the motor to run at about half speed, and the current consumed by the motor to double, thus accelerating the degradation of the brushes, brush wires etc.... This can happen in a very short time.....
Alternatively, a build up of carbon dust from the brushes could have caused either the aforementioned high resistance, or an arc internally which will toast the windings and armature.
OR, The brushes were wearing out, arcing, (or as Theo says, sticking) and drawing high current. (This can also melt the plug)
Any of the 3 possibilities can cause a motor to fail and will not, in most cases cause the fuse to blow.
A trip to the wreckers for a new plug, or a bypass connection on the bad pin and the new compressor, and you are good to go for the next 20 years.
Pull the old one apart and let us know what you find?
Cheers!