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What relay .. there is no relay.
You check for power at the 3 prong conector behind the booster. Red/blk is 12v power and y/blk is variable power [ 2-8v] for regulator trigger. If both are good, ground blu wire at regulator to complete the switched ground circuit. If blower works, reg is bad.

The regulator needs variable trigger voltage to complete the blowers ground side [ neg.]to operate. That is what the trigger voltage does.. it activates the switching transistor , which completes the blowers ground .. the hot side of blower is always hot . The switching is done by the reg on the GROUND side. So, by grounding the blu wire , you are taking the reg out of the circuit and will have direct/full speed fan.
There are NO resistors.