My 97 Astro trailer has some 'bad' (dead) lighting:
* All rear marker lights (the 3 in the back for 80+" wide trailers)
* Dead left brake/turn light.
The running lights (2 on each side) all work fine, and it's my understanding (sorry, I'm new to trailers and trailer lighting/electrical since I just got the boat in March) that the BROWN wire runs those, so that's good.
The RIGHT turn/brake light works fine as well (GREEN wire, from my understanding).
The tail-lights (headlights on, without pressing brake or turn signal on left) works on both the left AND right tail-lights. (BROWN wire)
That leaves the YELLOW wire (or its associated ground wiring to/from the light?) and 'some of' the brown (or associated ground wire) continuing along from the left tail-lamp to each of the 3 center-rear marker lights, as the suspects/culprits for my woes on the left stop/turn and 3 marker lights.
All that said, the wiring FAQ at http://www.etrailer.com/faq_wiring.aspx suggests that my problem isn't necessarily the 'Yellow' or brown wire itself, but rather a weak ground at the tail lamp assembly. Well, I've made sure it's not the light itself (tried the right side tail light (same conditions persist) and I've checked the ground which 'seems' to my amateur eye to be secure/solid.
Now, I'm not averse to replacing literally the entire wire loom and connectors. The connectors (where splices occur along the way) are at least 10 years old and very 'crusty' looking. Rather than troubleshoot just the left side via whatever method, perhaps I should just replace the wiring to/from the left side tail-light entirely with my own wire nuts and similar-gauge wire? I've done plenty of low and high voltage wiring stuff around the house, so perhaps this is my best course of action? Thanks for any feedback and/or methodical ways to pinpoint root cause.
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The drivers/rear lights on my trailer look like spaghetti under there. They must of daisy chained 5 or 6 different pieces of wire for some reason.. And if if I start messing with em, the light and the 3 markers will flicker, dim, etc.. Our trailers have the rivets for ground on all the light fixtures.. The 3 markers along the back were the 1st to go on mine.. I had to drill the fixture rivets out, and reattach with self tapping screws.. That fixed those. Then the passenger side rear trailer lights started acting up.. The parking light would be fine, but if I hit the brake or turned on the signal/ e-flashers (light should get brighter) it would just go out completely.. So, if I turned on my right signal, it would actually appear to work, as the light would just go out and come back on, instead of stay on and get brighter.. 
Only problem was when I'd hit the brakes while the blinker was on, that side wouldnt have anything.. 