>Using a 7805 would sound like the obvious answer if I had used one before,

>but I haven't and so I'm asking what would be the best way to convert 12vdc

>to 5vdc.

Try this site, half way down their is a circuit diagram for a 7805.
I have used it many times.  I would guess you could get the 12V from an
accessory supply in the fuse box. You might need to add a heat sink.

http://www.phanderson.com/stamp/intro2/intro2.html

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