No heat heat is not coming out proper it will blow heat out of dash and floor but not on the defrost for a 1999 grand cherokee laredo

you have a vacum line lose under the dash for the heater or the switch is dirty

Blend door problem under the dash

youve got a vaccume leak at the vaccume mode switch in dash or main vaccume sorce under hood.

sorry to burst you bubble, they are no fun to do, just make a note of every thing you disconnect and where it goes

The "blender" would refer to the blend actuators that change the flow of air from floor to vent to defrost or combinations. Properly working blend doors will allow you to direct air to the dash vents, the floor, the defroster, the floor and defroster or the floor and dash vents. If it is not working the heater will still work but you will hae no control oer where the air flows.

bad switch the cheapest solution or the blend door is not responding to the switch you can locate the blend door under the dash

Check the fuses under the dash on the drivers side, next to the steering column. There should be a fuse puller on the fuse box door. Also a chart of which fuses at what so you can go to the proper fuse. Good luck, Please let me know if this is the solution to your problem.

There is a vacuum leak under the hood somewhere. Check the vacuum at the speed control servo. If there is no vacuum on the line with the engine running, follow the line to find the leak. The vacuum reservoir is mounted under the battery tray. I would look in that area after confirming no vacuum at the servo.

probably a bad mode door or mode door motor

check your heater core