My 89 silverado keeps blowing the heather/ac fuse I have replaced the Blower motor resistor and the heather control. this is what happens

I reset the heather conrtol so everything is set to low fan and temp is in the middle. I do this by disconecting the battery ir just disconenecting the heater control then putting it back. I then turn on the truck every thing seems to work fine I can even go Days. i can adjust the heat and where the heat is directed . This is where i blow a fuse I hit the fan button to change the speed some times it will stay on for a few mins some times it will blow a fuse right away..That when i changed the resistor. still did the same thing so i decided I Disconect the blower its self just to see and it still blows fuses. that when i decided to try the heater conrtol got a used one and Still does it . Is there any thing else i can look for..OH and if the truck is on and ideling when the fuse blows it almost stalls the truck..

any help would be great please.
thank you