Door Side Light

[AgTires4295]

Before I start at square one, Im hoping somebody can enlighten me.

Project: A DIY light setup that turns on under the cab (shines down at feet) when door opens and off again when door closes.

Ive got all lights, wiring, fuses, etc but need to know which wire to tap before I spend time diagnosing which wire is which in the cluster. I know I can tap the wire directly behind the light in the door but am wondering if theres a better spot to attack vs taking the door apart. Any info would be great