Does the Evap canister pull a vacuum from the engine? If so, I think I know why my vents are acting up. The line is busted off of the canister. The canister would be right under the driver seat just on the outside of the frame rail correct?

 

AFAIK it does not, I just replaced one on a 96 Jeep cherokee with a 5.2 in it and before I did that the AC and Heat worked fine. I replaced it because it was broken it half, only the top part was still connected.

Search this forum for "wild vents" There is a writeup on how to fix those kind of issues.