A pack of plasters. Hate working on cars when ya cut yourself and have nothing to stop the flow so you can carry on.

I used to keep paperwork, etc, but since my car got stolen a few years back I realised how many bits the thieves got i.e. my name/address from my driving licence, a few letters, a £200 ring which I dropped in my car the night before it got stolen and couldn't find in the dark, a few spare car bits I kept in the boot, a Police trunchen thing (I had some trouble at the time ), so now I keep as little as possible in the car.