1998 24valve,
I just replaced my fuel filter and now my a/c is stuck on defrost.
I am sure it's my fault but I don't know what I did to make it do that and I don't know how to fix it.
I read on another forum that vacuum lines run the heat/defrost/vent switch... But I don't know where to start?

 

Classic case of knocking the main vacuum hose off of the vacuum pump. I've done it twice now, once while changing the lift pump, the other time a filter. Below the VP44 injection pump is the vacuum and power steering pumps. There is a 3/8ths inch hard tube for the vacuum system, usually black in color. Just plug that end back into the rubber boot on the vacuum pump. If that end is in place, check the other end and make sure it's up in the main vacuum tee along the lower edge of the cowling.

With loss of vacuum, you lose your ability to control the A/C vents, it will default to defrost. Your cruise control will not work either. And for the 4x4 owners, they lose the ability to control the central axle disconnect (CAD), so it will be stuck in 2x4.

Here's what to look for. This picture shows both vacuum and power steering pumps as they would be mounted on the engine. Vacuum pump on the left (aluminum colored), power steering on the right (black). The main vacuum connection you are looking for is the black rubber boot pointing straight up from the vacuum pump in my picture here:

 

there are some vacuum lines/tinny hose fitting at the top part of the firewall that I knock off and it does the same thing.

 

Yep, you were just playing with your fuel filter, vacuum lines run along that route and go to a T up on the cowl, drivers side.

Plug the vacuum line back in, trust me........ After 10 years of working on these things, it becomes second nature to check it every time you touch anything on the drivers side.

 

Thank you so much,
The vacuum hose had come off from the elbow that is by the power steering pump.