If it has the vacumm operated climate control check for a vacumm leak under the dash. Also check for proper operation of the vacumm valves operating the doors. Usually if the system breaks the control will default to the defrost position and vacumm leaks are a common cause.
If your windows fog up you thermostat may not register because you heater coil may be broken.
why my floor air comes on when the setting is at panel air on 2004 Chevy blazer
The heater core usually fails because the engine coolant hasn't been changed as often as it should have. The coolant gets acidic as it gets older. That and vibration makes the soldered joints in the heater core fail. To prevent this from happening it is reccomended that the coolant be changed every 50,000 miles or every 4 years whichever comes first.
The security light comes on after you changed the keys in the ignition from the off position to the on position as a confirmation that the electrical components in the dash board are working. It is nothing to be concerned about.
The compressor needs to be changed. There is no more pressure in it. It keeps working but only hot air is comes out.
The Izod Zealand blazer comes with four buttons, which is typical of most blazers.
a heater hose runs from intake to heater core to radiator. the heater core is were the heat comes from inside the car as well as defrost
Make sure your antifreeze levels are adequate (if it is too low, then there will not be enough hot antifreeze to flow through the heater core to make ther air warm.) I had this problem in my blazer and it fixed it right away. If that does not work, your heater core could be bad and a new one cost between 40-100 dollars and installation would probably be about the same. my ford puma has the same problem, i was losing coolant through the heater control valve on the bulkhead of the car so had to temp bypass it but if the valve stops working or the plug comes adrift it will stop warm water getting into the heater system in the car so i would check the heater control valve. you can buy them from eBay for about £12.
245/75r/15
4L60E.
If that car has a temp blend door actuator, it could be faulty.
Sounds like the heater core is plugged. Have a competent garage flush the radiator and heater core, then refill with fresh coolant. Should cure the problem.