How to correctly test voltage on taillight/trailer plug/harness?

Vehicle in question is a 2007 Toyota Rav4 Limited with the 3.5l V6 and has the factory tow package. I have a trailer harness in which the green pole, aka the right/passenger side turn signal, is staying at 12+V when the lights are on, and the voltage is not pulsing like it should be. The other poles are working correctly. I am trying to test the voltage coming out of the taillight harness/plug behind the rear passenger side interior panel, but I am scared of shorting something out.

I was thinking about putting the positive multimeter lead on either the red or green wire coming off the taillight trailer harness adapter, and putting the negative multimeter lead on the frame. Well first off, I’m not sure which is for the turn, the red or the green. Secondly, I am wondering if any of the other 4 white wires on the trailer harness adapter are a ground and if the negative multimeter lead should go on one of those instead.

Any help would be much appreciated. I have tried finding a wire diagram for this circuit, but I have had a hard time figuring out exactly what I am looking at in some of them. I can’t seem to find one that correlates exactly to the circuit I am trying to troubleshoot. I saw a post from years ago on here mention this site for diagrams, https://www.eautorepair.net , but they charge money. Are there no free options floating around? Also I figure taillight hookups are fairly similar and that one of y’all would probably know which lead is which.

Here are some pictures of the taillight to trailer harness/plug for the right side in question.