microcontroller powered by car adapter

Actually, the main reason I am replying, is to make sure that this
posting stays on top of the flood attack. (normally the spam doesn't
have any replies)

I am uncertain of the issue of voltage spikes or drop-outs which you
may need to contend with. But, I can say that there are low-cost
voltage regulator devices which should do the job for you in terms of
giving you the correct voltage.  Different Stamps take different
voltages, so I can't be more specific.

The easiest and cheapest way may be to buy one of the car cell-phone
chargers sold at a Dollar store, if you can find one with the correct
voltage.

Joe Dunfee

Buying a Y-jack or Splitter from a dollar store is also a good way to find
the right socket or plug for your needs.  Just cut up the cord and use heavy
gauge wire to extend the ends of the cord.  As for how to adapt a
microcontroler to 12 VDC use I have no idea.