It looks like I need to remove this to remove the door panel (trying to get at the rear latch mechanism). Does anybody know the size of the bolt head? It's really hard to get at, gonna need to buy myself some kinda driver, wrench won't get at it.

Thanks!

 

Deep 10mm should get it. Might be 8mm, actually. Just get a deep Metric set of 1/4" drive sockets.
When reinstalling shove a piece of tape in the socket then stick the bolt in the socket. The tape will keep the bolt from falling into the socket too deep and the threads not engaging.

 

Thank you ! Much appreciated.