I'm looking at the electrical schematic from the 2005 service manual and this is what I see.

You have two temp switches. One for the fan and the other for the high temp light. The fan switch is located at the bottom of the radiator and it has two connectors. One connector is for power and ground, the other one goes to the fan. The jump wire between the two connectors is the ground for the fan. The black wire on the connector for power that grounds to the frame has a diode at the end. Did your dealer check for a faulty diode. A toggle switch would not bypass this diode.

Just a thought!