I'll just use a voltage regulator, or resistor setup if I have to, but I am trying to ask if there is anywhere on the truck that is already brought down to 9 volts?
If the source is constant 12v (as in, voltage doesn't dip much under load) and you've got some diodes lying around, you can use them in lieu of a regulator. A silicon diode typically has a voltage drop somewhere around 0.7v. Put 4 of them in series and you're in business.
If you decide to use a regulator, something that looks like this will be cheaper and smaller than a switchmode DC/DC converter like the one q-ball posted.
you will probably find something similar if you crack open just about any dc voltage supply. the switched one you plug in to a car will have a resistor on each switch position to control the output voltage.
I built a voltage supply with one of those in junior college. use a potentiometer to adjust the voltage.
See if your camera supports above 9v.. Lots of the little mini surveillance cameras and video components, receivers, transmitters, hardwired cameras, etc.. that run on 9v batteries, always come with 12v AC/DC power supplies even though it says "9v" on the device it self. They may have their own regulators inside.
I would run a dedicated wire from the battery with an inline fuse. Then put a rheostat in line so you could adjust the voltage and lock it in. I wouldn't tap into the vehicle, simply because it could affect the ECM or other items in any particular circuit. Plus the voltage output from the alternator drops accordingly, when loads are not called for.