... but when I turned the air conditioner on max high, and held a little piece of paper in front of it to check for flow, the paper didn't move much. I suppose if I spray Lysol into it and can smell it coming out of the vents then, by God, I've found it.
Thanks for the tips, guys!
When doing your testing, you'll want to alternate the A/C setting between MAX and NORMAL. On the MAX setting, the external intakes are closed. If the stink goes away when the A/C is set to MAX, that would point to the air filter as the source of the odor.