sounds like it's interfering with the factory anti-theft system.
This link has some info on the alarm system, and how to add to it:
http://www.supras.com/06/techcenter/...3f54bb1e3d45df
The electrical system of the MkIII is a bit of a pain, I still can't fix my headlights... I'm pretty good at electronics, but the computers and relays are a bit complicated in these cars...