If you spliced in an overide switch then you are supplying power back throught the ignition part of the circuit. If you are splicing into the ignition power control for the fan you should also be disconnecting the wire that goes to the ignition source. You can do that with a SPDT switch. Put the fan control wire on the center pole, one side to ignition power, and the other to battery power (fused). Keep in mind, the fan will still only run when the fan T-stat tells it to. I wired my Black Magic fan to 12v all the time so it still runs after I turn off the truck. In hot summer weather, it will come on 2 or 3 times in 30 minutes after I park the truck. I never run in deep water, so I have no reason to kill the fan on command.