If you hook straight from the battery you won't melt a fuse unless the car catches fire. The circuit you hooked to will either be the battery side or the ground side. Unless there was a reason for the fan to be on the ground side should be open and normally expecting a battery voltage but through a resistance. If it had been grounded you would have known.

You probably did no harm, you were lucky! Never use an unfused jumper unless you really know where you are going and are prepared.

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