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A voltage reg is a trasistor that conducts power when a trigger voltage is fed to its gate .. vary the gate voltage and you change the output of the regulator, which is a switched ground for the blower..
You can also take the whole thing apart and ground the blue wire at the blower [ thereby eliminating the regulator from the circuit ] and that will put the fan into high speed ... but why do all that before you even know if there is trigger volts getting down to that mess [ I'm Lazy]

... kinda like pulling your starter out before you check the Nuetral Saftey Switch cuz your car won't turn over ..

I love that one " Hey man, it must be da starter .. my Uncle had dat trouble one time. yup,,"