IIRC the horn relay for that originally got power through a fuseable link, so unless your new kit replaced that with a mondo fuse of some kind, you should still have one. Did the kit come with a new horn relay? Old one could be bad (these days, so could a new one). If you push the horn button, the relay should click (fairly loud, yes, for that matter the horn should sound too, but then you wouldn't be asking for help
).
If the relay doesn't click, make sure you have power geting to it (got a test light?) then you can ground the wire that goes to the horn button, if it clicks then your problem is the horn button or it's contact (if the horns go off too, you may not hear the relay
but then, you want them to so be ready). I don't recall for sure which terminal is for the horn button right at the moment, but I want to say it's the middle one (someone chime in if you know please).
Of courese if it's not getting power to the relay you'll have to fix that first.
Other than that, the horn itself or bad grounds.