[ray_mcavoy]

The dim test light is an indication of extra resistance in the circuit that could be due to a corroded or loose connection. And the fact that it brightens up when you start moving wires around seems to point toward a loose connection. Are the wires you're moving the ones near where the harnesses plug together at the firewall? If so, I would suspect a loose connection at that point (possibly a bent / distorted terminal inside the connector that isn't making good contact).

The wiring for the brake lights runs from the brake light switch to the turn signal switch. Then from there to the firewall connector and on to the back of the truck. By passing the brake light wiring through the turn signal switch, it allows the turn signal to over-ride the brake light on the selected side. But it also means that a problem with the turn signal switch (or poor connection where it plugs in) can prevent power from passing through.

I'd recommend giving the existing under-dash harness a good thorough inspection looking for damaged / corroded terminals in all of the connectors. If you find a terminal or two that's bad, they can usually be replaced without much trouble. But if you find lots of corrosion or other damage, it would be a good idea to replace the harness. Although that alone might not cure all the problems if you're also dealing with bad switches and/or issues with the other wiring harnesses.