All I want to know is how does a blower motor relay switch work. I have a wiring diagram for a 1005 GMC Sierra and when on hi spped the connetion only goes from one side of the motor to a blower motor relay switch and I need to know how this is suppose to work . So basically all I am asking is how so and relay switch or coil work. Thanks

Do you mean 2005 Sierra? It doesn't matter... check all the underhood fuses. ALL of them.

Does it work on any speed? Or it doesn't work at all? If you don't have the lower speeds but HIGH fan works, the problem is probably the HVAC control head. If you don't have any speed, you'll need to find out why...

The blower motor gets everything it needs from the blower motor controller (kind of a computer of it's own). Battery voltage is provided (on the pink wire) to the controller but the voltage available to the motor is controlled (by the purple w/ white trace), and that's what controls the blower speed.

The speed request comes from the HVAC control head (the switches on the dash) When you select a different speed, the programmer "tells" the blower motor controller, and the available voltage is "stepped down", (voltage is reduced, to slow the speed), or it's not stepped down (allowed to flow, high speed)

When checking with a high-impedance digital voltmeter, at the blower motor controller, key on/engine off, you should have battery voltage on the pink wire (this will also be thicker than the others) The purple with white trace will be variable, depending on HVAC request. Make sure that works.

As often as not, you'll find a failed blower motor controller. But, if everything checks out electrically, try rapping on the blower motor when the A/C is on/blower motor on high. It may start up! If it does, replace the blower rmotor!