The trailer harness is already wired for a 2 bulb set up, basically, there are 4 wires in a basic trailer connection, White - Ground, Brown - Marker Lights, Yellow - Left Stop/Turn, Green - Right Stop/Turn.

Now im not quite getting what your saying when you say that when you unplugged the harness, the turn signals in the rear stopped working, but the brake lights still work?

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2001 Mazda B-3000 CabPlus "FX2 Off Road"| 3.0 | 5 SPD | 3 In Spindle Lift | Belltech Shackles | UltraGauge |Alpine Head / 10" Kicker Setup | Air Box Mod | Add-A-Leaf| Thrush Welded B4 Rear Wheel | Kobalt Box | Cobra 19 DX IV | Custom PA Speaker | Front/Rear Tow Receiver | 2X Aux Lights | Hideaway Strobes | Back Rack Safety Rack | Amber LED Strobes | Blue LED Strobes | 265/75/R16 General Grabber AT2's