if you are going to hook up a dc-dc, i would recommend that you wire it direct to the battery. you should place an inline fuse as close to the battery as possible, run the power to the PSU, and then run a ground wire to a metal surface of the car chassis or body. make sure you use a ground wire that is of the same size as the power wire.
running it off the leads from the cig lighter could lead to problems of pulling too much juice through the cig lighter circuit. you might be blowing fuses when you turn on the computer.
the inline fuse near the battery should be something like 20-30A. it basically protects the car from a fire that would result from a short circuit, if the wires ever got cut or crimped on something.
a nice feature would be an ignition switched relay that would only allow power to go to the PSU if the ignition switch was in the ON position or the ACCESSORY position. that way, the computer would only see power when the car is on. the same as your regular stereo. this can be easily done, but i dont have the time to explain it all right now...