Location of 7-pin distribution block?

I can not say for your trailer but all I have seen the underbelly is attached to the metal frame with self drilling screws. If you put the proper bit in your drill, they come out easily. The junction box shouldn't be very far in there. I have put back up lights on all of my trailers and every time, the one wire in the truck cord not connected to anything is the one for the backup lights but you should check it with a meter. What I do is run a wire from that box to the back and use small led lights something like this
They come on whenever the truck is in reverse and work great.