Looking good, I had exactly the same problem re the horn (2WD though), but mine was due to some numpty bodgeing the standard horn ring into the Momo wheel fitted when I brought it.

I noticed the wire for the horn isn't insulated for some reason, yet the switched side is, for this reason I taped over the connector as I thought it may touch the metal steering inside and activate the horn so check this first. Otherwise as said it's going to be the EARTH wire for the horn (Brown IIRC) it's switched on the negative side and so if this touches any metal it will activate the horn (This is akin to what the horn switch does when closed is complete the earth connection)

Also check the horn switch as where the two brown wires go in the switch if these move slightly it can touch on top of the metal round the steering.

Martin