Every tray loading iMac has a fan in it. That is Rev A-D, Bondi, Blueberry, Tangerine, Grape, Strawberry, Lime. It was a 90mm fan I think.. Just one size up from 80mm but if you have a round file(like the ones used to sharpen chainsaw blades by hand) you can make an 80mm fit as I recall.
You pull out the mobo chassis/carrier thingy and it is right there under the rear of the CRT. You can then take out two screws to remove it's cage. I cannot remember what sort of connector it uses though but I *think* I recall having to lop off an old one and attach it to the new fan.
Please note the direction that the fan pushes air in before replacing it with something else or even just removing it. (I have not reversed one myself, but would prefer to avoid it because it just seems like a bad idea.)