Where exactly is the ac drain line located?

The drain plug for the evaporator is clogged!

You need to flush out the drain plug. It comes out of the firewall opposite the AC system. It you get under the truck on the passenger side and look up at the firewall you will see a black rubber tube about 6 -8 inches long comeing out of the bottom of the firewall near the passenger side frame.

You need to get compressed air and blow it in this tube to free up the crud that is blocking it. you may have to repeat this process a few times depending on how messed up it is.

If you do not feel comfortable with this take it to your local dealer and have them do a full AC EVAP service, they have special chemicals to really give it a good cleaning.

You should also then clean your AC evaporator trap. remove the glove box and the metal wall behind it, then remove the AC resistor and get a vaccum and suck all the crud out of the evaporator housing.