must be a pinched, grounded wire

 

looking at a wiring diagram is very helpful. you can see how the complete speed control is by grounding the resistors or the high speed relay. a wire can be grounded anywhere/pinched,grounded.
+12 volts goes to one motor wire, the other goes to ground thru resistors or the hi speed relay mounted on the motor.
unplug the resistor assm. the fan should stop.one of those wires can be the one. if it runs slow its going thru the resistors.

get / download the svc info free ?somewhere