The seat belt light is controlled by the 'integration relay' on the back of the fuse box. This thing is pretty 'dumb' there is no real way to ask it whats going on.

The engine and airbag can store fault codes. The engine is easy to read with a universal OBD2 scanner. The airbag may blink codes out at you when you add a jumper somewhere.

But, since you have lights coming on from different systems, look at the battery and charging system. What else could be common to all these parts?

97 T100 4x4
97 328i
95 540i restoration project