Reverse lights wiring help

What wire do I tap into on the trailer harness to trigger my rigid ignites rear lights to turn on when I reverse. I saw some wiring diagrams say the middle white but I have 2 whites and I can't tell which one that is. One is thicker than the other.

I'm using this wiring diagram from @pwalnut.

Thanks for any help!

 

from the trailer plug ? a test light is your friend probe the wire with the reverse lights on and if your test light lights up you found it .

http://lamartrailer.net/wp-content/uploads/7-way-plug-diagram.png

 

Yeah, I'm thinking it's the thicker white one then.

 

It was actually the smaller white one. Took a meter to it to verify while the reverse lights were on

 

Thicker white one is the ground wire for the trailer.

 

Do you own a volt meter? Not sure the trailer harness has back up circuit.

I just tapped the actual back up light to drive the relay for my C3s.

 

Yeah, used the wiring diagram above. Put the truck in reverse, left it in gear and turned it off. Lights stayed on and put the meter on both to figure which had power.

 

If you have the factory tow package the cover is labeled