from Jeep44 post:
Did mine a few months back. I got a 90 with factory AC. First, remove the battery, battery tray and two coolant lines that hook into the heater core. Now remove the four nuts that hold the heater box in place. These are located on the engine compartment side of the firewall. Three are on the passenger side and the fourth is just to the drivers side of the valve cover. Now that you have that removed, move to the inside. Remove the four screws that hold the AC unit into the dash. There might be more, hard to remember. The 4 are located at the bottom of the dash. Now you can lower and move your AC unit around. Careful not to break any lines, just work it slow. The heater box will slide out. It is tight, so expect to make some turns with it. Now switch the blowers. Enlarge the hole in the firewall with a dremel or jig saw as I did. Make it big enough so you will not have a bad fit, you only want to remove it once. Now reinstall everything. Enjoy the heat. I also installed a 190 thermostat for winter. My YJ gets so hot that I rode around with a t-shirt and the fan on low on a 30 degree day.