Relay engine compartment 86 Turbo

[Goosh]

I am trying to sort out the auto-heat system in my 1986 911 Turbo and have some questions (I am not very savy when it comes to electric stuff)

I can't seem to find out what the black round relay in the engine compartment relay box is for - see attached photo - I do know what the others are for, but can't find documentation on the round one.
Also, what causes the burning around the 25amp fuse with the red wire attached - looks like it gets pretty hot!
The bottom white fuse, which I think is an 8 amp, looks to be for the passenger fan and according to the stamping on the cover it should be 16amp - wouldn't this fuse blow all the time?

Thanks, Goos

[john walker's workshop]

oxygen sensor computer relay. dirty/loose connections get hot.