Tow bar electric fit backfired and none starter!

Good evening guys and girls,

I have a very beloved 2002 Audi A4 (B6) Avant TDI Quattro and have just melted it's brain!

I bought and fitted a tow bar, all fine, then subsequently fitted the electrics from the 7 point hook up. This was done by uncovering the wire internals from the light unit (3 lights each unit, 6 cables for in-out current), through voltmeter testing each cable I was able to identify which were which, the cable end from each 7 point wire was then connected to the unit cables, integrating them into the circuit.

HOWEVER, once this was done I jumped in and proceeded to start the car up, slipped the key in and turned to 1st stage, loads more warning lights came on that were previously not (car passed MOT three weeks ago with no electric faults and has been fine until I touched it), the ESP, handbrake, ABS and central locking dash lights to be specific. Then when trying to crank it, absolutely nothing (battery giving 12.4v out), then when turning back to stage one ignition the lights remain on and a persistent electronic ticking occurs and rev, speed, temp and fuel dial hands tremble! Nothing works, I pressed the hazard light button and it blinkers as if AC is going through it opposed to DC.

So, it is apparent that this prognosis has come about by me attempting to wire up the tow bar/light fitting interface where I proceeded to connect and disconnect three cables, it looks and sounds to my untrained eye that it has shorted and caused some damage somewhere to the cars brain, I love this car and very much need it for work and my sporting life so if anyone could guide/help me out I would be genuinely appreciative.

Thank you in advance.

Brad