WoodsJustin

The car is actually an SLK55 AMG 2012. It has the trickle charge/jump start points under the bonnet.
The battery was dead (I forgot to leave the trickle charger connected).
I attempted to jump lead the car, positive first, but then there was a large spark when I connected the negative.
The car then would light up the dash, but nothing else would work - mirrors did not open up, the lights and menus would not function.

I charged the battery back up and still nothing.
I connected my OBD reader and the car immediately came back to life.

I can only suggest that the more modern or expensive Mercedes have a safety protection mode.
Hope this helps someone else, as I could not find this information on the internet.

MJ50

moved to R172 AMG forum...

ChrisHimself

Try a fresh battery, sometimes a battery is super super dead. Worse you would have done was blow a fuse/relay. You can absolutely fry a harness but id be uncommon, thats more like a sustained short to ground which melts the harness over time.