According to Wikipedia, some 2nd generation Ford Explorers were equipped with 3rd row seats beginning in 1997. I'm wondering how different these seats are from those found in 3rd generation Explorers, and whether it is possible to install the 3rd row seat from a 3rd generation Explorer in a 2nd generation Explorer. If so, what sort of modifications (if any) would need to be made in order to install the 3rd generation 3rd row seats in the 2nd generation Explorer?
It is not reasonable to do. 2nd gen had a raised rear deck with the spare tire stowed under the vehicle. Any add-on 3rd row seat would have to be for short children only.
3rd gen the floor is lower instead of flat deck all the way back behind the 2nd row. It just won't work unless you were to cut the legs off then people are essentially sitting on cushions on the floor.
Plus, there would be no access to it. The 2nd row folds down but without the lower floor panel you couldn't step in to get to the 3rd row, would be more like a cross between a dive and a slither to get there.
SO if you want to do it, you need to cut the rear floor pan out of a 3rd gen, cut the floor pan out of a 2nd gen, weld it on, and carry only an emergency spare if that (or roof carrier, or rear gate external) because those crucial few inches to stow it inside behind the 3rd row, are not there on the 2nd gen.
The modifications necessary will be all the fixings points and relevant strengthening - even possibly built-in spacing.
Depending on the age of your second gen one, it may already have the fixings if it was one of the last built as the "new" panels may have already been included in the build.
If nothing is there then, as these have to support forces applied in a crash - will the insurance accept such a modification or will you avoid telling them...