Quick investigation[hopefully more in depth tomorrow weather permitting] as to why the horn don't work on my Volvo 960 showed that the original horns have been replaced at some point by a 3 horn airhorn which has rusty terminals-which can't be helping matters-checked fuse anyway which is good but then it came to the relay.Schematic on underside of lid shows where/what for the relay positions are but not colour of relay,it also ascribes a letter to each relay[A,N J etc]but on the car all the relays apart from one at the very front of the box are black and all have the letter J on top.The one exception being yellow with no letter on it.Everything on the car works-apart from the horn of course but how do I tell if the horn relay is the correct one or if I can temporarily swap it over with one of the others to prove it's not a relay fault?