Where is the rear defrost button on suburban 2007LT

You have a short to ground in your rear defrost circuits unhook your rear defrost on both sides and see if the ground is eliminated at the fuse. If so it's the grid itself, if not the hot wiring is shorted going to the glass.

You have either a blend door problem,or the blend door actuator. Both are located underneath the dash.

ck ground

Use the same glue used for the rear-view mirror. Its available at the autoparts stores.

We just had the same problem and the AC, rear defrost and recriculate switches stopped working as well. We shut the Suburban off and when we turned it on again the issue was resolved. It hasn't been an issue since.

Auto parts stores have a kit to repair the connection.

I have a 2004 suburban that had wiper/defroster problems. I would check the connector between the hatch and the body. It is located at the top right corner. The plug is under the rubber boot where it enters the body. Mine had lost connection due to corrosion and needed to have a jumper wire placed around the connector for the power and ground wires. Give it a look and see if that is the problem.

At Radio Shack they carry some Heat tape just for a Situation Like this.

if it light up check connections at back window & door 

This does sound normal to me. When the defroster is switched on, the AC engages to help the defroster be more efficient by acting as a dehumidifier before the warm air hits the windshield.
It is possible that the AC system may be over charged with Freon. But, I must put my faith in the mechanic and that he is honest as many are and has done you well.