What exactly are you trying to tap in the cab? Most of the wiring under the dash is somewhere between 18-20 ga.
Then the 18-22 ga ones are the ones you want. Just make sure that if you mess with the switch in the door jamb that you are really careful taking it apart because the springs under the plastic cap in there are easy to lose.
The wire gauge used depends on the amount of current drawn by the load (whatever you are controlling). For example the wiring to the heater blower motor will be a lot heavier than that to the dome light. When adding on to a circuit, never go heavier than the existing wire. In your case, 18-22 should be fine (unless you're lighting your footwells like a stadium). And the color of the wire doesn't matter.
I'll be doing a full write up once all the parts have arrived.