This....
Or drill a weep hole
Every vehicle we've had suffered from this issue at varying degrees with the AC. It's a pain.
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This....
Or drill a weep hole
I tried the foaming stuff on an Escape we had with A/C that stank. Didn't work. What did work was spraying about 3/4 of a can of lysol into the cowl intake area while the A/C was running. I'd switch it back and forth between full cold and full heat in between blasts of the lysol. The lysol smell went away after a few days. Never had an issue with it again.
Turn your blower to max and run your hand along the cowl. You should feel where it's getting sucked in. The best way would be to pull the cowl plastic and spray it right in the intake hole, which will be obvious with the plastic off. Not sure how hard it is to take off the cowl plastic on a 15+ F150 though- some Fords are cake, but some have a bunch of clips that can break and are impossible to find replacements for.
I did the Escape at a car wash so I could spray the Lysol residue off the plastic cowl after I was done.