T&T: 12 volt receptacles

[John Tones]

Tim - there is a series of 110 volt plugs and sockets that are referred 
to as "workshop 3 wire" and are foolproof for your needs. They have the 
regular pin set up like the common household units except that one prong 
is at 90 degrees to its mate. They will mount in the same space as the 
110 volt units you now have but you will never be able to mix them up.
Home Depot carries these devices as well as the cover plates (at least 
they do in Canada) and they are cheap in comparison to the other 
suggestions that have been made.
Hope this helps you out of your bind.
YMMV

John Tones   "Penta"
Sidney, BC