There is a special issue with 12VDC wiring, and that is excessive voltage drop. This goes way back to the argument between Nikoli Tesla and Thomas Edison.
Unlike AC circuits, DC circuits have a voltage drop you may have to contend with. While you won't likely see any issues with a low-current appliance - especially if it is LEDs, it's something to be aware of every time you wire in DC.
The thing is that the DC wire must be sized for voltage drop, not current carrying capability.
And the length of the wire matters as well. The longer the DC wire, the higher the voltage drop.
Fact is, voltage drop is a more significant issue than the ampacity capability of wire, and you will find you will likely need to increase the wire size due to limiting voltage drop far before you will have ampacity issues.
The way to do is to figure out the ampacity of the branch circuit (existing) as well as any added ampacity requirements of the new wiring, then determine wire length and AWG. This will allow you to determine the voltage drop in the wire.
For non-critical circuits, you can usually get by with no more than a 10% loss in voltage along the wire.
Here is a voltage drop calculator, that works well for both AC and DC circuits:
Voltage Drop Calculator
Be sure though that you use the calculator to determine round trip distance (of course if you are using the chassis ground, you can use the one-way distance as the ground is so massive that it eliminates concern about voltage drop).
I am providing this info for you for future use as much as your present project. If you are using LEDs, then the current demand will not likely present many voltage drop issues. But if you used incandescent lighting, it could be an issue as a 6 bulb fixture could draw 10A @ 12VDC.