The blower motor is controlled by three wires. Wire 1 is ground, wire 2 is high mode, and wire 3 is high mode. You need to wire these in such a way that the two modes can never come on at the same time. I don't know what colors 2 and 3 are, but I can nearly guarantee that ground is going to be black. You can trace them back from the motor itself, however.
The controls on the back of the console I believe aren't related to the fan at all. There's a couple places to plug in a headphone jack, a few mechanical 'switches' to control the vents, but not too much else, really.
Here's the schematic I would use. It's intended for use for an electric fan like what would cool an engine, but a fan is a fan is a fan.
Just replace what would be the temp switches with a DPDT switch. Something like this would work fine, although any DPDT (double pole, double throw) switch would work just fine. Rocker, toggle, etc.