Is 12V in car same as in computer power supply?
Yes you can get 12 V from your PC power supply. But you must know how to start it outside of the PC (the PS_ON pin).

Also, most PC PSUs need a load on the +5V voltage to fully start up.

However if you use the PSU while in the PC box, there's no need for load and PS_ON, just be sure to do the wiring with all the devices powered off, to avoid ESD.

For your camera, you need the black wire (GND) and the yellow one (+12V).