Fun fact I learned the hard way, I don't have barn doors though. The metal clasp in the door that grabs the hook can get jammed and the door gets stuck so oil that metal clasp. I thought my lift gate was locked, but it was jammed and took a big pull to unstick. The door lock actuators do start to go after 20+ years, my dad's 2003 express is like 50/50 on every door if it will lock. My 05 suburban is more like 90% chance the doors lock lol. Sometimes a lock only will move halfway and look locked but it isn't.
For rear defrost check the windows resistance with a multimeter, if it's not open circuit then check the contacts for voltage (while defrost is on), if there's voltage then the relay is good. You're hoping it's the relay basically. Does your top brake light work? I think that's on the same harness.