The top section bolts to the firewall from the outside in. In order to get it out I'm almost certain the dash will have to come out, at least part way. I put an evaporator in an 05 Yukon and it took about 6 hours total but I've had a few NBS dashes all but out before too so I knew what to take loose. Its no fun thats for damn sure. Is the evaporator leaking or what seems to be the problem? Evaporators almost never go bad. I had to replace the one in the yukon because the customer said the AC smelled when the AC first turned on. I hope you have a better reason then that.