HELP! What's a Disc Modulator for a 1993 MX6

[glenakers]

PLEASE HELP!

My girlfriend's MX6 heater went tits up a last winter. Started blowing hot then cold air, then slowly progressed to no heat at all. Air conditioner blows COLD, and changes to outside air (not warm at all)when AC is set to "warm" setting.

I took the car to the Mazda dealership and the guy told me it was a "disc modulator" inside the dash that regulates the heat and AC. He told me it would be 600 bucks to fix and that the AC will not work either because the disc controls that, too. I told him the AC worked fine, and he said it won't real soon.

Well, it's the middle of Summer in Texas and the AC is fine, but of course the heater still doesn't work. I have tried to find out what the heck this "modulator" is but have had no luck.

HELP.

Texas