trailer wiring

[Cruiser]

I've spent all day trying to reconnect wires and nothing works! A mechanic got the lights working when we brought the trailer home. Now after new floor and insulation are in, I'm installing new wiring. I marked the hodge-podge of old wires to reconnect as before, but without success.

I looked at the wiring diagrams off MarksRV website and found that the GM wiring code seems to match what's on the tow vehicle: white-ground; yellow-left/brake; green-right/brake; brown-running/tail lights. I tried variations of these in the trailer, but no luck.

Two things confuse me about the existing connection.
1) The brown wire from the car is connected to two wires in the trailer, one for running/tail lights and one for brakes. The brake and tail lights appear to be wired to the same bulb. The turn signals each have a dedicated wire and bulb. This is different than the car where turn signals and brakes appear to be wired together and running lights are separate.
2) From the car, in addition to the flat 4-wire, there's also a sheathed cable w/ 10ga black and white wires (from the brake controller?). The white wire together with the white ground of the 4-wire are connected to the two trailer brake wires. Does a ground exist for lights if the white 4-wire is going to the trailer brakes? The black wire isn't connected at all. We were told a wire was installed to connect later to charge the battery, so I assume this is that wire?

Is there a procedure for connecting trailer wires, or is it by trial and error? Does it work to try one or two connections before connecting all the wires, or does the entire system need to be in place first?

I hope that this makes sense to someone who can give some advice or direct me to resources. Boy, I'd tow the trailer to the RV store tomorrow if I could move it. Trying to figure this out is the hardest part yet of restoring this old trailer.

Doug