Your fans seem about right current wise. The resistor should be able to handle that load. Where did it burn through? Was it at a connection?

Look at some figures: V=IR. During high blow you have 25a current. Figure 12.5v and the usefull resistance of the motors is .5ohm. The resistor is usually .2 ohm so the circuit with the motors and the resistors will have .7ohm and the current should be 17.85a. Now if you go straight to ground (say because one of the screws is too long and goes to the fender) then the circuit is 12.5=.2R, R=62.5a. Which one do you have?

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician