defroster works now, not later....

[gramps77]

Sometimes I have to slide the lever back and forth then other times it will just start working on its own after about 5 minutes. I replaced the selector levers with a new aftermarket one from LMC. I didnt replace the vacuum line box that goes on the back of it. Are those prone to cause this issue or does the truck just need to warm up before the defroster starts blowing air out the defroster vents?