The Gen II (92-95) and Gen III (96-99) mast tops are different. (I converted my 99 to a power antenna, they were not available in 99) Gen II has a "diamond" shape on the end, and the Gen III has a flat top design. I an not positive about the motors, but I think they are the same, maybe the mountings to the car are different.
However, the Gen II mast
will work in the Gen III housing, only difference is it will retract about 1/2" below the level of the housing when fully retracted though. I had installed one in an extra Gen III antenna I have and used it before finding a spare Gen III antenna at the boneyard. The Gen II mast'd antenna is still in my garage on the shelf, I put the Gen III mast'd antenna in my car.
Someone will say in the next day "get a xxxxxxx fixed mast antenna and you won't have to worry about that anymore."
I prefer the power mast. I can remember to put it down if my car ever sees a carwash.
***WARNING*** There is a guy selling them on eBay from "calcar" do NOT buy from him. I bought a mast, never saw it, and he won't refund any money. I talked to three other people that bought from him, and one of them said it took EIGHT MONTHS to ship a mast. He doesn't have them, and waits until he gets his hands on one- one of them was even apparently used.
If you need a mast, get it from Bridgewood Motorsports- they're also on eBay, great sellers. The Gen III masts however, are difficult to find.