why does my heater only work on defrost on 2005 Grand Marquis,AC works the same way

I had the same problem on my 1998 grand marquis. I had to replace my climate control.

Temperature control modulator. YouTube how to bypass it. Super easy

Have a 2003 Merc Marq. LS with climate control. Same problem. Dealer told me new module, for $600 plus install. Researched and found out the design was bad because of a weak chip inside module which controls air flow, etc.It fries when interior gets too hot. Bought a used module for $100 and put it in myself. Worked fine for 3 months. It fried, same symptoms. Found a company in Van Nuys, California replaces chip and remans the module the way it should have been built. That was 5 years ago and haven't had a problem since. Company is Module Repair Pro. 5 stars in my book.