Ignition switch failures.

[toolboxchev]

Test it on the high setting, you will find that those things draw as many as 40 amps or more depending on what is feeding them.

Higher output alternator is not helping also.

Since completely redoing the underdash wiring harness I do not run the fan on high. Have tried with a 10,15 amp fuse and I keep blowing them so..............

All the fan motors are coming from 2 sources now, and they are rated for both AC and non AC applications. Problem is the non AC truck wiring is only good for a 10 amp motor. Do not know about the aftermarket wiring so it sounds like you have to update that to accept the higher amp motors.