I just did this mod to my SR80i a few days ago.
I used the stock cable, a 1/8" horizontal mount stereo jack with screw end, a 1/8" stereo plug (both from Radio Shack, but there are better options out there), 24 AWG copper speaker wire.
First, I took apart my right cup. I cut the wire connected to the right driver right around the zip tie (using my method, you don't have to desolder the stock wire from the right-sided driver). I removed the nut from the jack and took a measurement of the threaded end. Using an equal-sized drill bit, I opened the hole on the bottom of the cup to accommodate the jack, then threaded the jack through and locked it into place. The corresponding slot on the outer half of the cup I widened with a drill bit and a razor blade to accommodate the size of the locking nut. When put together, the nut is flush with the edge of the outer half of the cup. Then I soldered the positive and negative wires from the right driver to the appropriate tabs on the jack.
I cut the main wire off right below the Y-joint, so all I had to do was solder the 1/8" stereo plug to the end of that (utilizing the stock plug on the other end). At this point, it's smart to use a multimeter to figure out which segment of the plug is used for right and left so that it's consistent with the other end.
For the left cup, I drilled a 1/8" hole on the top of both the right and left cups for the speaker wire. Before soldering anything, I fed the speaker wire through the headband, using a little bit of dish soap on the wire to make feeding it through a bit easier. It's a pain in the butt, but doable with a modicum of patience.
I desoldered the stock wires from the left driver and soldered in the speaker wire in the appropriate places after feeding it through the new hole in the top of the cup. I then fed the wire through the top of the right cup and soldered it to the appropriate tabs on the jack.
This was just my way of doing the whole thing on the cheap -- all the hardware cost me under $20. There are obviously other ways to do it with higher grade wire or better hardware. Also note that this was easier for me because I don't have any screens on the back of my cups.
You can check my profile for pictures of the results -- specifically an over shot and then a shot of the right cup.
