Nothing unusual to have an engine cooling fan maintain the engine running after shutting down the truck. Earlier post described the fan turning from a motor to a generator when the ignition is shut down. As the fan looses speed the voltage generatored by the fan back into the system also dies down so in turn does the truck. Possibly to test that this is the case disconnect 12v to the heater fan and sustitute a test globe. Check operation again using the globe as a load instead of the heater fan. Having wired numerous hot rods this generation effect is not uncommon some do some don't. To overcome the problem I have used a diode from a wrecked alternator in series with the 12v to the engine fan. Solder some wires to the diode you may have to file the top of the diode before it will accept solder. Cover in heatshrink and mount safely. Heater should be wired to the accesories position on the ignition switch via a fuse to the heater also mentioned earlier but some installations do not have an acessories facilty. If the run-on effect is due to the heater motor the install of the diode will stop this . You will need to ascertain which direction the current flows thru the diode with a meter as it will only allow current flow one way. If you haven't got a meter just make some temporary connections by twisting the wires together. one way it will work one way it won't. I am suprised the a heater fan motor would do it but with electricity whether it be 12v 110v 240v or 22000kv nothing suprises me.
