My heater in a 99 2500 4x4 works off and on. I changed the themostat the other day didn't help. The heater will work sometmes. Any suggestions?

 

Check the vacuum lines that are in the area where the heater hoses go into the firewall. A vac leak will keep the switch from operating that determines hot or cold air. Might as well check your other vac fittings while your at it.

 

Most likely cause for no heat is low flow through the heater core,either plugging or low coolant level in system.

Bob