Update for A1278/13" MBP: This post is a guide to remove the subwoofer assembly from the wiring harness.
You will need a sharp, pointed instrument to remove the pins from the plug that mates to the header on the motherboard, as well as a cutting tool to cut back the heatshrink tubing. I used a pair of flush cut/trim snips used in trimming component leads in electronics for both purposes.
The subwoofer wires are the two most right black and red wires (looking at the wires with the fan above them orientation)
First, use the sharp tip of the tool to press down on the raised metal tab of the pin (gold color) while pulling back simultaneously on the wire in order to remove it from its housing. Repeat the process on the remaining wire. Make sure to pull the wire as close as possible to the plug while also pushing against the plug at the same time; this is so the forces cancel each other out, as you don't want to damage the plug or header on the motherboard.
Next, trim back the heatshrink tubing slowly and delicately so that you can separate the subwoofer wires from the bundle. The heatshrink that you are removing is what bundles the subwoofer and right speaker subassembly wires together. The subwoofer and right speaker wires each have their own heatshrunk bundle, do not cut into those! Unfortunately, I don't have a 'before' photo of the heatshrink still on the cables.
Unscrew the subwoofer and remove it from your MBP.