I did three in our 78 Ambassador a couple of months ago...there's power at each opening...
I used a heat gun to loosen up the old sealant, and it came right off. I used a plastic scraper, not a metal one to avoid any gouges in the alum shell.
The then exposed rivet heads are easy to drill out.
You then can pry off the old vent...I then used the heat gun again to scrap off any remaining 'goop' left on the skin...
It's then a simple matter to 'square up' the openings to allow the new fans to mount in the opening...I used a large bead of sealant on each side of the 'gasket' provided with the fans, then screwed it down with the provided screws...then another bit of sealant on all the screw heads...no leaks...
Go inside, hook up the
12 volt wires, and bolt the interior 'guts' in place to finish off....
Have fun....