If you want a light to come on from either or both of two feeds, you can often use two diodes.
Connect the anode of a diode to each of the feeds. Connect the cathode together and to the +ve feed to the light. The negative of the light connects to ground.
However, that assumes a few things.
1) Either feed can provide enough power to run the light in addition to what it normally runs
2) Both feeds turn on and off by switching the +ve.
3) The extra voltage drop from the diodes won't cause a problem.
Obviously the diodes have to be rated to take the current that the light takes.
Without knowing the details of the circuits you can't be sure if it will work. If there are no lights that come on when the tail gate is lowered, it is unlikely that there is a feed that does what you want.