Don't cut the box, that's tacky. just ubolt the dash, very easy, and swing it down. I had mine completely removed for a dash swap. This will apply for all mustangs with a/c
TOOLS and supplies NEEDED:
-7/16 socket, preferably long socket, short extension and ratchet
-7/16 wrench
-5/16 short wrench
-5/16 socket and ratchet, preferably long
-Black RTV
-phillips screwdriver for door sill and kick panel removal
-3 to 3 1/2 inch socket to jam between dash and firewall for clearance
-Beer
1. Remove dash
2. A. Remove the nuts on the firewall on either side of the air conditiong accumulator. For this I used a 7/16 deep socket and short extension on both sides. Pull the bracket back on each side. Don’t be afraid to pry and bend the bracket back and away from the firewall, it won’t break, and it’ll bend right back on.
B. Now remove the nuts that were behind the bracket. I used the same 7/16 setup from before on the right side, and a 7/16 wrench on the one closest to the passenger side.
C. While you are there, take off the heater hoses going into the firewall and spray the heater core outlets with a white lithium grease so they will be lubricated enough to slip off the rubber grommet.
3. Pull the carpet back slightly on the passenger side. This is important for several reasons. The first is that coolant may spill on your carpet. The second is to gain access to the bottom housing nut seen in the following picture.
4. using a 7/16th socket, remove the very bottom housing bolt shown in the above picture and the top two housing bolts shown in the following picture.
5. Grab on the entire box and pulling it towards you, wiggle it out a few inches. When it’s out far enough the heater core outlets and studs will be on the inside of the firewall as shown in the following picture. Do not attempt to pull the whole unit out, it will not come out all the way with the air condition condenser still installed. The goal is to just pull it out just far enough to gain access to access the bolts on the top of the heater core lid.
6. Jam a socket between 3 and 4 inches long between the top of the heater core lid and the top of the firewall as shown in the picture. This is essential or you will not have any room to access the back bolts.
7. using a 5/16th deep or 8mm (it is possible to use a short socket but it’s more difficult) undo the front two heater core lid bolts. Now use a very short 5/16th wrench to undo the driver side rear bolt. Now use the deep 5/16th with the small ratchet to undo the passenger side rear bolt. Please keep in mind that these two rear bolts are a pain in the butt to remove, take your time and do not drop the bolts behind the back of the heater core box. Additional tip is that if you do not have enough room to pull the rear two bolts off, you can remove that plastic insulation that goes along the top of the firewall/frame.
front two bolts
rear passenger side bolt
8. Remove the heater core see picture. Be careful not to spill coolant.
9. Remove all of the old black silicone from the inside of the heater core box. Make sure all of it has been removed, this is critical
10. Put a bead of silicon(I used the black rtv) around the edge of the inside of the heater core box where the new heater core will rest, as show in the following picture. Lubricate the new heater cores outlets.
11. Slide the core in while a friend makes sure that the outlets slip through the rubber grommets at the firewall. The rubber grommet does not go through the firewall, it stays on the inside.
12. The heater core is now seated properly as you can see in the next picture. Bolt the lide back onto the heater core box. The passenger side rear bolt is not essential if you are having trouble putting it back on.
13. Pull the socket that’s jammed in there out. Push the box through the firewall with a friends help from the other side to watch. DO NOT TUG ON THE HEATER CORE OUTLETS.
14. Put the three nuts back onto the housing and put the nuts back onto the studs coming through the firewall. Bend your brackets back, put your final two nuts on.
15. Reinstall dash.
