Topper 3rd Taillight Wiring

Recently purchased a Leer topper that included a wiring harness; however, I haven't been able to find a matching one, so I bought one from O'reilly's. However, I'm having difficulty determining which wires match up, as they are different colors on the original wiring harness, the tail light assembly, and the new harness I purchased.

Can anyone shed light on how to match the various colors?

This is the original harness I will be replacing:

These are the taillight assembly wires:

This is the new pig tail:

I'm pretty certain I will not need all 4 pins on the new pigtail either, so which should I use?
Planning to wire my dome light direct to battery, so disregard any wires that would generally be used for that purpose.

 

Multimeter..?

 

That's totally different than my ARE I bought used. Mine only has a positive and negative from the cap. I just simply grounded it to the body and tapped into my brake light wire.