'05 GT A/C on, 2 Vents Blow Cold, 2 Warm

[ModVic281]

I just changed the cabin air filter on my brother's 2005 Mustang GT Sunday and all was good. Yesterday, he turns on the A/C and it is only blowing cold air through 2 vents and the other 2 are warm.

I'm thinking that maybe it's the blend door actuator? I see that there are multiple ones for these cars. How do I find out which one controls the hot/cold change and the correct part number?

Thanks!