I had to do a heater core replacement, and while I was in there I decided to take the rest of the AC out since it wasn't functional anyway. So what I did was remove it and painted a piece of sheet metal black. I took some self tapping screws and put them through the box and the sheet metal and then took some silicone sealer and sealed up all the cracks. I had my blower working at one point and it held pressure with no problem. If you have have to do a heater core in a Monte, take a look at Ray Buck's site for his Heater Core Replacement manual. It walked me right through it with no problems.