1998 grand cherokee limted control stuck in defrost setting

the vacuum supply source is not there check for vacuum under hood to dash. on passenger side by firewall.

We have replaced a lot the heater control assm's Tom,The three knobs for your controls is the part itself. its only held in by four screws. They charge $25.00 at our salvage yard for them . the worst thing would be a door on the heater box is stuck, that would involve removing the whole dash.

The LH models (LHS, Concorde, New Yorker, Intrepid, Vision) do have self-diagnostic capability with the standard type heater controls. I don't know about the Sebring heater controls (my '95 manual doesn't mention it) but if you have the various controlknobs/buttons that I will describe you could give it a try.
There is a rear window defogger that is initiated by a push button with an LED light. That light is the readout, by its blinking. To begin the diagnostic mode, run the vehicle at idle with the controls set as follows: the blower speed knob should be on any position other than OFF, the temperature knob should be set in full COOL, and the mode knob should be set on Defrost mode.
To enter the diagnostic mode, press the defogger switch in for five seconds, until it begins to blink, then release it. It will continue to blink for about 30 seconds while it calibrates. If it goes out, the calibration is OK and you can skip 3 paragraphs to "Position...mix mode" below.
If the light stayed on, switch the mode knob to the MIX (defroster-floor) position. If the light stays on there is a problem with the control head unit.
Switch the mode knob to the FLOOR position, if the light stays on, it indicates a problem in the blend door or the blend door actuator wiring.
Move the knob to the Bi-level (panel-floor) position where a continuous light indicates a problem with mode actuator or circuit.
Position the knob in the MIX mode. The LED light should now flash twice then pause, and continue a two-flash sequence, indicating that the MIX mode is OK.
In the FLOOR mode the light should flash three times, meaning that the FLOOR mode is OK.
In the Bi-Level Mode, a four-flash sequence indicates that mode is OK.
In the PANEL mode the light should flash 5 times to indicate no problems there, and in the recirc-panel mode 6 times and seven times in the recirc Bi-level mode.
The temperature control can be checked by switching the mode knob to Defrost and rotating the temperture knob. If the temperature contol is OK, the light will flash faster the more you turn the temperature knob up, until the light goes out at the hottest setting.
Switch the knob between Panel and Recirc-Panel and watch the recirc door (unit under the dash). Proper operation means that circuit and components are OK.
To get out of the self-diagnostic mode, turn off the blower or push the air conditioning button once.
If this doesn't seem to apply to your control head or you get no response from an LED light for the rear defroster, then the Sebring may not have this capability. Please let me know your results if any

remove switch wires fried

something might be stuck in the ducting not allowing the door to move and direct air flow to the other ducts. First pull straight on the nob and pull it off of the dash. then take a pair of pliers and make sure the smell metal peg will rotate.

The HVAC blend door is stuck common problem in jeeps.

If you have the manual a/c heat system the vent controls run off engine vacumm. If it goes to defrost when accelerating then you have a vacumm check valve that is faulty, if it is a permanent problem you may have a broken or leaky vacuum line or faulty control head(unit with knobs, ect)

Most common cause is a vacuum leak The blower door actuator is vacuum driven and a small leak would be enough to prevent the door from moving to blow air from the vents.

selector mode switch is bad

YOU NEED TO CHECK FOR A VACUMM LINE THAT HAS FALLEN OFF A CONNECTION, OR YOUR VACUMM MOTOR THAT CONTROLS THE BLEND DOOR HAS GONE BAD AND NEEDS TO BE REPLACED