Horn and hazard light

Intermittent short somewhere ?  I have  a vague recollection of some wire behind the dash rubbing and getting worn through on some bracket (sorry I can't be more specific, maybe a search will find where it was mentioned).

That particular chafed wire caused a non-start rather than your problem. One other good possibility is the J519 under the dash on the drivers side. They aren't cheap though! It should show a fault but not under the engine ECU. There is more about it in the list of 'how to do's'.

I don't have the link at the moment but if it is the one I'm thinking of it won't be your problem anyway because it was a big red unswiched 12v supply cable that chafed through and shorted to earth coming close to a fire. When you said you had 'plugged it in' you need to look at every system the van has. If you can find someone local with VCDS diagnostics they can do a complete scan for you which might give you something to go on.  The J519 unit I mentioned is known to cause problems and does have a part to play in both the horn and the hazard lights. I wouldn't change it without being sure unless you can get a cheap second hand one. It does need coding also.

Have a look under the driver`s seat, pull the carpet back and find a wiring splice, check it for corrosion and poor crimping. This splice causes no end of electrical problems, all well documented, get some crimping connections and make it good. 

Sounds like it might be related to an alarm.  Any chance your van has an aftermarket alarm fitted ?

Alarm battery on the way out ?

My money is still on the J519.  The alarm has separate siren under the fresh air intake grille outside on the drivers side and is a different sound from the horn.  The alarm siren is known to go off randomly if it fails but not usually in conjunction with the hazzards.  I agree that a wiring fault could cause your problems but I would investigate the J519 first.  Sometimes there are no codes (wherever you look) despite an obvious fault.