[Hozer 88GTConv]

The FSRM states that the 1988 GT convertible is approximately 136 lbs heavier than a 88 GT.
The weight comes from a variety of sources.
Primarily from the heavy frame of the top assembly. The hydraulic motor that sits behind the rear seat is no slouch either. The slave cylinders are fairly light, but it all adds up.
The verts have a stiffer motor mounts and an "X" tubular underbody frame attachment to the k-member. The A-pillars are also reinforced.
If you are looking for more specifics, let me know. It's not all in the same place in the manuals, but the exploded diagrams of the top assembly are nice.
Any Ford dealer can print it out for you too in their parts department if they are nice.