These wires are giving me an amp-le headache

Oh Boy, this is a BIG QUESTION with hours of discussion possible... I'll try to simplify a bit:

There are 3 electrical systems in a vintage trailer.

1. TOW WIRING is not as difficult as it seems.

a. There is a ground, often White. Easy to trace, as it will be connected to the frame somewhere. Most little trailers do not use a ground wire at the light, they use the skin instead. So, a screw thru the metal light back plate into the skin is the ground.
b. Tail Lights and side Markers (TM) are usually all one color, Green is common, sometimes brown. For instance, say the TM wire is pink, then pink would run to both tails and all clearance lights.
c. Each turn signal has one wire. Say Red for Left Turn, then Brown for Right Turn.

There's really only 3 hot wires to worry about: TM, RT and LT. Your trailer "might" use black for TM, but I can't tell where it is actually ending up. If it goes to each tail and the marker lights, probably it is TM

2. The 12 VOLT SYSTEM is the inside trailer lighting and other fixtures running off a battery. Again, white is often Ground. Hot wires can be any color as can the ground in a vintage trailer.

3. 110 ELECTRICAL: obviously for outlets and some lighting. Not really what you're talking about here.

I'm seeing multiple connections that seem to be inside the wall cavity. They need to be crimp connections.
The little blue crunch things do not last and cause many failures. Wing nuts are never acceptable inside a wall... In one picture, you've got way too many wires to be tow wiring, so I think a PO must have run all the wiring in one clump.

It would be easiest if you rewire the entire trailer. Start with TM and run to each tail and marker. Many of us also run a ground to each light. (White)

Then run a separate wire (different colors) to each tail. They are your brake lights and turn signals.

Junction boxes are really handy and make future wiring troubleshooting very easy. You might run the back tow wires into a J box at the back. Makes if very easy to connect existing (or run new) wires.

I hope this has helped you. PM me if you need more help or post here.

Wow! Thanks a lot for all that info, I think my mind has done a revolution of wrap-age. 

I have a question about the lights that are near the top of the trailer, on the front and rear. Do all these lights, 5 on both front and rear, behave the same? The previous wiring looks to me to be the same for all, where I thought the far left and right would be indicators. Are they just running lights or clearance lights like you said vikx? Leaving the only rear tail lights as indicators? 

Also is this connection to charge the on-board battery while towing? If so do either of you know what this connection is called?



It leads to two wires and one (positive?) continues to then be wired throughout the trailer for various lights, the stove fan and light etc. and then finally ending up connecting to the battery like you mentioned vikx. Should the 110 setup be able to charge the battery? I would have thought so but I don't see any connection for my trailer. 

I realize rewiring it all might end up being the best option but all the wiring is there and I really want to just understand the system first so maybe I can realize nothing is too gone and salvage most of or all of it. It sounds like it will be wise to replace some of the connections too.

Thanks a ton for the help! I would be lost without it