On a 123 with ACC III,(temp wheel on left, 5 buttons across, fan on right) there is a blend flap behind the glove box. it has a green vac line going to it.

remove the glove box, you'll want to replace the disintegrated foam tubing anyhow, and check to see if the flap is open or closed, and operating correctly.
Mine wasnt closing, ever, so I disabled it.
to permanently close it, you can simply unhook the vac line to it. That is what I did. then I melted the end of the vac line w/ a lighter and pinched it off. if I want to revert, I can simply cut 1/4" off the line and plug it back in.

also check the monovalve- it will only take a few minutes.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.