I have a 1996 GMC K2500 power staying on to my heater fan

Either the blower switch wiring has been altered, or you have a short to power in that circuit. The dash controls are not supposed to have power without the key. Are there any other functions that stay on with the key off? Stereo? turn signals? This would indicate a problem in the ignition switch, which is where all power must go through first.

replace fan blower motor, it is beginning to fail, carbon pile brushes in motor are worn or motor stator is worn.

Did you remember to replace the fuse?. So, the fan kept running even with the controls in the off position? Were there any other accessories that would run with the key turned off?

I the fuses and relays are good for the pump it could be a bad pump especialy if you was having starting problems make sure you have power to the pump. Also do this before changing pump. Put your key in turn it on dont start wait 10 mins remove key wait 10 seconds repeat this a total of 3 times. If that dont work try a different key do the same thing. This relearns your key to the ignition it started with Gm cars and trucks from 1999 to to 2006. It is a big problem but there is no recall that I know of. If that dont work a dealer needs to probably replace and reprogram your ignition switch and keys

Are you familiar with using a 12 volt test light between + battery cable, and post of battery to find it? Takes a little time, but good way to find draw.

You have to remove the stering colum covers and in the ingnition key drum on the side there is a little hole, press the inside pin and pull the key ccylinder out, you can buy on autozone or pep boys.

Ignition switch is bad

If the heater blower motor resistor failed then the heater will stay on full blast to keep your window clear, for safety. If the key is turned off and the fan still blows then I suspect you have an ignition switch problem. Main power comes from the key switch to feed the heater system. The plug at the ignition switch could be melted and making contact or an internal switch problem. The switch is located on the steering column under the plastic cover.Unplubbing the switch should help diagnos the problem.

fan motor will not shut off

key cylinder pin that holds it in is stuck replace key cylinder they will give you new key along with this