What would cause no heat in front but heat in rear on a 1999 navigator?
This would be your AC Blend Door causing this problem. It is a door that seems to be notorious for breaking in ford/lincoln... Ours is doing the same thing and I haven't replaced it yet. You can google "Lincoln Navigator AC Blend Door Replacement" and there is a guy out there that has pictures on how to remove your dash in a ford forum. The dealer will charge anywhere from $1000~$1900 to fix this. You can get away without taking the dash apart if you go to this website (www.heatertreater.net) and do it his way using his part (he has videos). I would only perform this if you are scarred to turn to a wrench and have some good organizational skills...
Blend Door Problem
Your problem sounds like the common
blend door problem on the Ford Expeditions, and Ford Explorer. The
door is on a vertical axis and when it breaks, it can swing back
and forth randomly giving heat or AC or nothing. The difference on
acceleration is the force placed on the door swinging it one
You can check the operation by fully opening the glove box, removing the actuator motor on top of the plenum box and examining the blend door axle (the actuator is under the unit on top of the hump behind the flexible black plastic cover. It is on top of the unit, behind the glove box in the 2002 F-350 though). Pictures of the common fail signature are available on the Related Links below. There are several "free" fixes that mostly are worth what you pay for them. The solution on the web site is solid and proven over hundreds of vehicles and will resolve the problem once and for all.