Who understands wiring diagram???

[Kneale Brownson]

I have a circumstance where I blew the 5 amp fuse for the left running
lights trying to set up for wiring for a trailer (with a relay device that
takes power from the battery).  Replaced the fuse and no power to the left
running lights circuit.

So I'm following Tony Hoffman's advice that the circuit goes back to the
headlights switch and there may be a problem either with the switch or
somewhere else in the wiring.  Anyway, at the switch, there are two wires
that get powered when the headlight switch is set to the first notch (tail
and front marker lights), one is solid gray and the other is a green/black.
 Since the left rear tail light is a gray/black wire, I presume it starts
from the headlights switch as the solid gray one and the right is the
green/black, but I can't tell from trying to read the wiring diagram.

I have a headlights side of the combo switch that I swapped out of our V8
(same device), and when I check it out with the ohm meter, the two posts
that "light up" when the headlight switch is first activated are the top
outer post and the third down from the top inner.  Those are, respectively,
the gray and the green/black wires in the harness.

I have a spare bit of headlights switch harness that I plugged into the
switch on the car and powered up (either of the big red wires) with the ohm
meter to find that both the gray and the green/black wires are turned on by
the running lights position of the switch.

So I presume my switch is OK and I have a failure somewhere else in the
wiring that's making the left running lights circuit nonfunctional.

Since I want to use the left side as the source for my trailer tail lights,
and since I apparently can't use the existing wires, I'd like to tap into
the switch harness and run a wire to a relay and then out to the left tail
light.  I'd make the wire from the relay 14 gauge or 12 gauge and have it
provide power to the tail light wire in the trunk and to the harness for
the trailer.  I'd power the relay from the fused battery source (12 gauge)
for the relay unit that works with the brake/flasher lights.

Can anyone tell me which wire (solid gray or green/black) at the headlights
switch is for the left side?

And does the rest of the plan sound reasonable?