how would you "normally" power the dvd player? Via a wall wart? If so, what's its output voltage.
Does it come with a car adaptor? If so, does the car adaptor step down the car's voltage?
Trying to determine whether it is, in fact, already powered by 12 volt power sources. If so, no harm using a 9 volt source as well (i.e. if it'll run off of 12 volts and off of 7 volts, you shouldn't harm it running it off 9 volts. OTOH, if everything else is only supplying it with 7 volts, 9 volts could fry it)