heater/defrost won’t switch from defrost to heat
Check the vacuum supply line from the intake manifold to the vacuum reservoir and firewall. If there is no vacuum to the control panel the system will remain on defrost.
also it could be the switch itself. my son has the same problem in his ford work van. when it happens he changes it and it goes back to normal
I assume this truck has an EATC panel? If so, vacuum leaks inside the EATC are a common problem due to a number of O-rings, etc which have a tendency to fail with age. Stuck on DEF is the main issue but there are others.
There are internet and YouTube guides and tutorials on doing this repair yourself and there are companies out there that specialize in rebuilding them. Dirt cheap as a DIY project but having it done can set you back a hundred or two plus the downtime,
if any under hood or under dash work has recently been done, look in those areas for a vacuum line that has been broken or knocked loose.
not that I’ve ever done that…