Ok... I have a 1999 Grand Cherokee Laredo, when the vehicle warms up turning on the headlights will turn off the blower motor... turn the headlights back off and the blower motor works again... have replaced the blower motor resistor... and talked to several mechanics... all seem to be stumped... any advice on where to start?

Get a hammer and punch. Tap on punch next to key while turning key. This will free up frozen tumblers in ignition

Plenty of causes. Plugs, valves. rings, numerous electronic/electrical faults. Diagnose vehicle and eliminate problems one by one.

You might still need to take in for service.

There is a resistor mounted by the fan, this could have burned out. Look on passenger side right by fan motor should be 2 screews that have 5-6 wires pluged into it had to replace mine once fan only on high worth looking in too. Also, check if the blower motor Resistor Harness was faulty.

Good luck.

The 1996 Jeep Grand Cherokee Laredo Anti-Theft System is equipped with Vehicle Theft Security System (VTSS). VTSS system monitors vehicle doors, hood, liftgate and ignition. If an attempt is made to enter or operate vehicle without proper disarming of system, vehicle horn will sound, headlights, taillights and parking lights will flash, and engine will be prevented from running.

The following wiring diagram and electronics circuit contains detail schematics of The 1996 Jeep Grand Cherokee Laredo Anti-Theft System. Herein you will find interconnection between electronics parts of the system such as body control module, auto headlamp light sensor. VTSS led, driver door module, passenger door module, cylinder lock switch, junction block, power distribution center, and grounding. Herein you will learn also how the Vehicle Theft Security System works, removal, installation, testing and troubleshooting procedures.

Try HERE.

Also, remember that here can find the Jeep Grand Cherokee Service & Repair Manual.
Hope help with this.

2 problems?
No blower motor: Blower motor resistor very common or motor itself or heater controls.
No heat: Start simple with check antifreeze level after vehicle has sat for a few hours. If that's OK then it could be thermostat, heater controls, blend doors, water pump, restriction in cooling system

Was the battery disconnected at anytime? You may try locking the vehicle up, putting the key in the tailgate to reset the security system.

OK, if this motor is totally dead and, it will not operate under any circumstances, this will confirm motor failure. normally, if the switch is faulty, you would get some uncontrolled action form the motor. it would blow at the highest level all the time but, if its totally dead, you will need to replace the blower motor in this case for sure.

Your daytime running light module is burned out. Unplug or replace it. Usually its located near/ behind the passenger side headlight.

it could be that the heater is air locked. joe