They take a 2 door Golf or GTI, saw off the sides of the car, as well as sawing off the sides of the Jetta, and weld the the GTI side on the Jetta.
It is basically the same operation as reparing a badly side impacted car.
On the MK3 they use the GTI rear window, although for the MK4 the window is too high and was custom made. As for an MK2, no welding or body work required, all you have to do is find one :bigup: