Fan not turning on

[Nilao]

The ecm supplies ground to disengage the fan...you would put the switch in the ground circuit to interrupt the ground to turn on the fan. The issue that will arise doing it that way is the fact that the ECM eill wer that loss of voltage and throw a check engine light as it sees that as a faulty solenoid or circuit. If you dont mind a check engine light all the time then by all means do it that way. They only way to be able to make the ground switchable without throwing a code is to suppress the code via c@lt3rm. The simple way is like chamberplains mentioned, add a second solenoid that you control via a switch to release the pressure.