There are several possible reasons for a "melt down" Perhaps the initial solder joint was "cold" or the joint/wire was damaged in handling. Perhaps a few strands were broken in handling. Increasing the resistance at this point would cause the joint to heat up.
More importantly, running at or near stall current will really cause things to heat up! (I used to sell a precision current calibrator that could put out 100 Amps continuously and it could melt contacts and wires far bigger than what we are dealing with here if the connection was not perfect.) Worse, you will destroy the motors if you are not careful. Treat these babies gently if you want to have a reliable robot.
I agree with all that the plastic brush holder and etc. in the motor is extremely fragile...one of ours's has a hairline crack already. Be careful of how you mount the motor if you use the brackets in the kit! And don't drop one.
