Yes, you will see. You will not be able to get all of the sticky stuff off, but don't need to. Be careful and don't punch a hole in the EPDM (or rubber). When going back on you will need butyl tape and it is very very sticky. When it sticks to itself, you can't get it apart. The trick is to keep it cool when working with it. The first time you accidentally let it touch itself, you will see. The other thing is to have a tube of the right color dicor so you can cover the joint where vent meets roof and the screw heads. Make it look like (or better than) the factory.
One of my vents locations had a cheap-0 fan, so the wire was there; the other wasn't wired. IMO, the factory should wire every vent location. I had to pick up B+ and ground from a nearby switch and light. The source you find... make sure it is "hot" all the time, not just when a light is on. You can work 12v "hot", it won't hurt you, but you might want to find a remove the fuse, so as not to blow it. If you see Romex nearby, that is 120vac... leave it alone!
While up there (be careful up there), give the whole roof a good looking over for bubbles and places for potential leaks. I found a few.
Oh, I love my two Maxxair vents. One of the best mods I can think of.
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A Modder's work is never done!