Isn't a 1998 model a 993?? ( even on your side of the pond
)
IIRC, a 996 doesn't have a seperate control module for the spoiler, but is controlled by one of the main computers in the car.
If it is a 993 you have, the spoiler module is located up under the dash near the glove box. (about 2" x 3" x 1" thick )
max