How to activate car mode with a magnet?

The Google Nexus 7 tablet has lots of reed relays and magnets. Some toggle the screen, others activate the wireless charging dock. I suppose there is a position for activating car mode (never putting screen to sleep). I'd like to put a magnet (or several if necessary) into my car dock, so I can trigger car mode, whenever I put the tablet in there. Unfortunatelly, I couldn't find a page, that will describe the various positions of the reed relays inside the device.