[Galaxy S III Accessories] s'pht:
My aim was to make a desktop dock that has audio out. No need to hook up the headphone jack to the top of the phone. My other aim was to get it working on my S3 and my Wife's S2 (Epic 4G Touch to be precise).

A lot of the info I used came from http://cleanimport.xda/index.php?threads/1754102/

Parts:

I used the ZENiS desktop dock http://www.ebay.com/itm/110893901462 Paid for it on July 31, it arrived Aug 9. I actually bought two of them because I'm making two of these docks.

I wanted to get going on making the dock so instead of waiting for my local *REAL* electronics store to open, I ran over to radio shack and got a resistor multi-pack http://www.radioshack.com/product/index.jsp?productId=2062303 Great thing about resistors is you run them in series and they just add up.

While at Radio Shack I also grabbed a two pack of stereo plugs http://www.radioshack.com/product/index.jsp?productId=2103452 Big win - these can be flush mounted and make the whole thing look professional-ish

A little bit of wire. I yoinked a couple inches of CAT5e from a spool and pulled out some individual wires so I could hook up the audio plug.

Multimeter. I think I got the one I used free with a coupon http://www.harborfreight.com/7-function-multimeter-98025.html Couldn't find my auto-ranging one. The multimeter is so you can make sure you're getting the right resistance.

1/4 inch drill bit. For the audio plug hole.

Soldering iron and solder.

To start off I opened up the dock and found that indeed it was perfect for this: lots of empty space, exposed solder points, conveniently labelled pins. Perfect. Now using my multimeter I went through the resistors in the pack and put together a combination that came *really* close to the 365k ohm needed to trigger desktop dock mode. While at it I also made one that's 619k ohm so I can make one of these http://cleanimport.xda/index.php?threads/1321491/.

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Car dock resistors on the top (4) desktop dock resistors on the bottom (3).

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I did quite a bit of testing to make sure I was getting the right pins, right resistance etc... In the end you simply solder one lead of the resistors to pin 5 (the ZENiS pin labels are accurate) and the other to pin 4. You can do it on either of the little boards. I chose to do the resistors on one board and the audio on the other. The "tip" on a stereo connector is left, the middle is right and the bottom is ground.

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The 1/4 inch hole I drilled in the casing. The audio plugs have a thing you can screw off then on to hold it very tightly in place. Works perfect.

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My Galaxy S3 (with extended battery and in a case for the extended battery) fits perfectly. It also fits with the normal battery/back/case. My Wife's S2/E4GT also fits... if she takes off her case. The E4GT is wider than a normal S2. I guess my next modification is to widen the hole just a tiny bit so hers will fit in the case.

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Front view. The orange arrows are pointing to a little on-screen button that switches the phone between audio out over the dock and audio out over the phone's speakers. I'm assuming that in the latter it would go out over the normal headphone jack if I had something plugged into that.