The relay has four terminals, One wire is constant live, there is a second live wire which I think switched live via ignition meaning it only has power when the ignition is switched on, another wire is the live wire to the fan.

This leaves one wire the relay earth which is the wire you say goes off to the themostat this is where the fan switch is and it connectes to this and the other wire going to this switch connects to earth.

Someone has obviously messed with the wiring before as the relay should be in the fuse box along with the rest.

It may be the fuse has blown which protects the fans if the original setup is as shocking as the Cosworth's to test you need to check you have +12V at two terminals with ignition switched on.

I assume you know the fan should only cycle if there is insufficent natural airflow, I.E when stuck in slow moving traffic.

The interior light problem defo sounds like a fuse, no relay is involved, have you checked all the fuses?

Martin