You definitely need the fan working otherwise you could lose your engine, the relay pictured controls the fans. There are four wires, two will be lives one permanent and one switched or both permanent depending on weather fans are under ignition control, don't know on these cars.

This leaves two wires, one is the ground for the relay which goes via the fan switch which is on your thermostat housing hence the wire which goes over there wire, and the other is the live to the fan, when fan switch closes this trips the relay hence switching fans on.

Normally yellow wire on the head unit is the constant live and would have blown the fuse this is probably why the interior light no longer works as there on the same circuit.

If you are competent with electrics, all your problems are DIY!

Martin