If its Eatc, it is likely the blower speed controller. If is is manual temp control, likely the blower motor resistor. Both do the same thing, just the blower speed controller is a solid state part.And the resistor is simply that. Both located in the heater plenum behind the glove box. The only high speed fan relay I know of is for the cooling fan, and its located in the Inegrated relay control module. Blower motor speed is only contolled by the two parts I indicated above. When they go bad, they default to high speed.