Help please, Electrics in Freedom!



Last night ..... mmmmmm. Fast asleep and then some b*ugger's car alarm goes off. 1.20am. OK someone get up and turn it off. Get out of bed and have a look. Thats ever so loud. Its from one of mine! Sh*t! Must be the new Fiat, rush downstairs, get the fiat zapper, push the buttons, lights flash, alarm keeps going. Get dressed go outside. Its coming from the Day Van. That doesn't have an alarm. Get the Van keys, open the doors, start the engine. Horn still going. Open the bonnet, rip off (and I mean rip off) the wires from the battery plus 4 other wires broken in the process. Horn still going.

Get to leisure battery and rip (again rip off) the cables from the battery. Silence. So sorry to everyone who had to listen to the noisy additional horn for about 10 minutes (just glad it wasnt the musical air horns still attached!) .

So, tonight, go to van, reconnect wires, fortunately they were all different colours so I think i have then connected correctly, reconnect earth wire to battery, and Van starts and all essential lights are operational. Then, after about 30 seconds, the horn goes off again, so under the van and disconnect the horn, silence again.

Reconnect the leisure battery and all lights seem to work, and music is back on.

Multimeter onto the red wire going to the horn, 12.7v. How is that live? Re check the volts and after about 2 minutes the volts start to drop, down to zero volts, but slowly, as if a capacitor has been put in line.

So this lead has, all by itself made itself live, and it seems to be able to store power, for a short while.

Sensible answers please, and take the p*iss if you like, but I really need some ideas here. No, no other electrical additions have been made in the last few months, before anyone asks. This one has me a bit bamboozled! Help please.
Horn switch sh1t itself causing it to be constantly live?
In-line relay done above?