You're right, the seatbelt lights don't work, and can't either.
I've just connected up the heaters in the UK seats... Which leads me to another couple of questions: -
What are all the wires for on the passenger seat of a J-spec?
After connecting up the heaters on the UK seats, I'm left with 2 unused wires. Both white/black, one with silver bands. I assume these relate in some way to the seatbelt light. One rests on 9 volts when the ignition is on, the other rests at earth.
If you apply 12 volts to the one resting on 9v, a relay, (or something that clicks), activates inside the drivers rear quater panel near the seatbelt location. You need to ground the 9v wire to disengage the relay... What is it?