I have a 2003 chysler Sebring and my rear defrost will not work. When I push the defrost button, I hear the beep it makes to turn on but the rear window never defrost.

Your car has about 40 fuses. Which one are you lo0king for?

The only thing they have in common is the ignition switch.

Is yours a coupe, sedan or convertible? If it's a sedan or convertible, there's acalibration technique you can run. With the engine running, turn the temp all the way to the cold side. Turn the mode all the way to the defrost position. Put the fan switch to any position other than off. Then press the rear defroster button until you hear a chime. Let it go. The system will go thru some gyrations and give you a "report" in your odometer readout. A "0" is good.
You may have a bad control head. As you noted, the default mode is defrost, so if the body computer senses a loss of signal from the controls, it will put it into defrost.
Again assuming yours is a convertible or sedan, salvage control heads are fairly commonly available on ebay and in junkyards. The part is used in the sedan and convertible and in the Dodge Stratus from 2001 through 2006, so there are lots of them out there. It's an easy replacement which I would definitely try before tearing the dash apart.
The IOD fuse is the one in the middle of the fuse panel at the left end of the dashboard. Easily recognized by the "ears". You can pop it out with no tools.

You don't

You won't have the knowledge or equipment needed
to leak test & pull a vacuum & recharge the system

Also-- you pollute the atmosphere for a number of reasons

Need a repair shop that does a/c

Have you checked the refrigerant pressure with gauges? If not, do that first. If there is sufficient pressure but you are still getting hot air, your diverter door for your air handler has stopped moving, probably due to an vacuum leak under your dashboard.

it should be a little round button located on the heater control assembly,below the AC button

I ahve this same problem. My car also bogs down when going over 2500 RPMs at certain times. My car was diagnosed with needing a new ECM. I have not had this replaced as of yet.

All those buttons normally are on the center concole. The heat/air knob turns to the red for heat, the more its turned in the red the hotter the heat blows. If your talking about the AC air, that should be on the knob that lets you chose the fan speed, just push it in to turn on the AC a light on the button may come on to let you know its been activated. the Rear Defrost is a single button, i think with 3 wavy lines top to bottom also in the center concole.

there may be a defrost window/ liftgate fuse under dash or in the engine compartment check that first/ you can also check the plug in for the defroster it just pull out and clean with steel wool/ after that if that does not work then the actuall switch is bad switch the button that is/ if you have questions let me know Please rate my response thank you very much

You have a wiring harness problem
Take it to the dealer directly.......