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well, crap. Spliced green pair of wires together and purple pair of wires together into new speaker....and... nothing.
...
what did i miss?
The OEM subwoofer is a two channel amplifier driving a dual voice coil speaker. The purple pair is one output, the green pair is the other output. You've just shorted each amplifier output together in your terminal crimp. Hopefully you didn't fry the amplifier. Use a dual voice coil speaker and wire it properly.
 

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It would appear that the OP connected the two amplifier outputs in parallel which is a bad idea. The two amplifier ouputs are stereo, one for left, one for right. When there are differences between L and R, one amplifier will have a different output than the other. If the amplifier outputs differ and are wired together, then a large current will flow briefly while there are differences and lead to distortion and waste a large amount of amplifier power and be hard on the amplifier output transistors. Most music has little stereo content at low frequencies so sometimes it will seem OK.

To be more technical, the reasons not to parallel amplifier outputs comes from that each amplifier has a separate negative feedback loop that corrects the output signal to match the input signal multiplied by the voltage gain so that an output error gets negatively amplified to try to force the output signal to be correct. With a second amplifier directly connected to the first one, then the two will both fight to make their separate left and right signals correct but the outputs are being forced to be identical by the short circuit of the two channels in parallel. This is bad. Don't try to run one speaker coil with two amplifiers.

If the two amplifiers have their input signals first going into a summation amplifier to sum L+R to mono at the amplifier inputs (it may have this), this will minimize the signal differences and improve efficiency driving a dual coil speaker, but still there are slight differences in amplifier gains and response due to component tolerances so a dual coil speaker is needed if both are to be used and get the power of both amplifiers used. Two separate speakers would be great too but would take too much space.

When you use a dual coil speaker, then each coil produces a separate magnetic force, but are electrically isolated. The net sound wave is the vector sum of the two signals. Putting both amplifiers in parallel on one voice coil doesn't get you any more output power anyway since the voltage gain is fixed, and one amp by itself can easily drive the 4 ohm load.

If you are going to use a single coil speaker, you'd be better off just using one of the amplifier outputs. The original poster was incorrect about the coils being in parallel, they are actually on separate amplifier channels. Also incorrect that two 3 ohm loads in parallel makes 6 ohms (it would make 1.5ohm).
 
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