The trailer connector works just fine except for the +12V power

There are a couple things that can cause this, but I am thinking you have either a wiring issue if you have no 12 volts on the red wire or you have a ground issue.

Here is a guide that will help with this testing, but we need to check the ground and the wire's resistance:

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Please see the wiring diagram below as there are a couple more fuses that are related to this system, but I assume they are ok if you have power on the other circuits.

So that means we need check the resistance of this red wire and then check the ground.

Let me know what you find, and we can go from there. Thanks