S40 Horn not working
 

I know the fuse and relay are under the bonnet. The fuse is OK but I can't budge the relay..

Anyway, is the ECM in the passenger footwell involved at all with the horn?

I know that the ECM needs fixing so perhaps this is causing the horn problems (as it did with the wipers!)?

Otherwise, I can just buy a new horn/pair of horns !

The ECM is in the passenger side wheel arch.

The CEM is under the glove box.

Not sure if either have any connection to the horn?

EDIT - it's the ECU that's in the wheel arch.

CEM is connected to horn. Not directly maybe but sends electronic signal to horn when button is pressed. Thats why you can activate horn, wipers, even sprinklers from laptop with Vida. And also why if CEM goes out, all weird things happen
OP - you may be on the right track, inspect CEM first