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Yeah..not the best way , but someone has been there before you and they have done a short-cut [ probably to save a few $$$ ], but as long as they have just grabbed the Positive supply side and it is still fused , I would just go for it.
As long as you still have variable trigger , then you still have CC speed control..so that is the important part.
If you really want to get to the cause of their jumper remedy, you will have to trace the circuit with a schematic for refernce..that is the only way to truely track it down..[ unless you luck out and see some burnt wires/connecors somewhere.]
Go for it and keep an eye on the pos feed fuse/wires. You may also want to remove the fuse and throw an amp meter across those terminal just to make sure that full blower is not exceding 25/30A, just to verify any bad wiring in what they did...............................................