I just wanted to keep you up to date on what I have found.
Dark Green - (circuit 1619) TRR TRN (fuse in under hood) - Right Park & Turn Signal
Yellow - (1618) TRL TRN - Left Park & Turn Signal
White - ???
Brown - (2109) - TRL PRK - Parking Lamps
What should be somewhere, close by, is a light blue wire, which would be used for the stop lamp. You may be able to follow the trailer wires back under the chassis and find this light blue wire. That would be where you would have to connect the topper light.
If you can not find that wire close by, you have a choice of:
1) Running a long wire into the underhood fuse block area, and tap into the fuse block where you should have a fuse socket (and possibly a fuse) labeled TRLCHMSL (right next to the one I have highlighted in the diagram below).
2) Getting one of those "center mount lights" that they have at the auto parts store that comes with a little electronics box that will prevent the center light that you are installing from flashing when either turn signal is used.
I hope this helps. Please let me know if you have more questions.
Take Care, Greg A.