I don't know the RST intimately to know what it is, but there should be a legend on the underside of the cover, or failing that the handbook would tell you.

Also you can see the fuse is melted as well, this may be the supply that is switched via the relay, clearly someone has not bothered to replace the fuse. This needs doing immediately!

There is a good chance this is the heater motor, although relays are not usually used.

Does the cooling fan work?

Regardless, it will have to be rewired externally now.

The reason this happens is due to a heavy consumer, that melts the fuse box when powered on for a while.

Martin