[Galaxy S III Accessories] FreydNot:
EDIT: I just noticed the pictures aren't loading for me. I'm not sure if it's like that for everyone but just in case you can see them over on imgur.

I got my setup all working today. I am using a Samsung Infuse car dock that I modified for OTG capabilities and USB data pass-thru. I hooked that up to a Behringer UCA202 audio interface which lives in the center console of the car. Conveniently, the center console has the audio aux input and a 12 volt power socket.

I used a 15ft USB A to USB Micro cable along with a 1.5ft USB Male to Female Extension cable as a make-shift OTG charging cable. I used a 6inch 3.5mm Stereo Plug to 2 RCA cable to connect the line output of the DAC to the car audio input.

(Click a picture for a full resolution version)

First you have to pry the cover off the back of the dock. I did this by running a small flat blade screwdriver all the way around the edge. You have to pry around each of the catches to get it to release.

There are three catches along the back side and two catches along the bottom. You can see them in the above picture. Take your time and be careful and you should be able to pry it off without doing too much damage.

The circuit board is held in with two small philips head screws. Remove them and the board easily comes loose.

I used 30awg wire to connect pins 2 and pins 3 of the two USB connectors. Those are the blue wires. There isn't much exposed pad to solder to on the external connector (labeled J3). It takes a steady hand, a small tip on your soldering iron and a good magnifying glass to get the job done. This is definately not a job for someone with little experience with soldering. Once the wires were attached, I put a blob of hot melt glue over them to provide physical strain relief (it really sucks to pull the wires off the connector when you strip the other ends).

R1 and R2 are two surface mount resistors in series (100k) between ground and pin 4 of the micro USB connector that mates with the phone. 100k between pin 4 and ground tells the phone it is connected to a car dock. A dead short between pin 4 and ground tells the phone it is is connected to an OTG cable. I removed R1 (R2 doesn't do anything once R1 is gone) and then added a wire between pin 4 and ground. I would have preferred to do this at the other end of the micro USB cable that plugs into the dock, but all (most?) standard USB cables don't pass pin 4, so it must be done at the connector that mates with the phone.

After testing everything to make sure it works, mount the board back in the dock and snap the cover back on.

To make the cables, start by cutting the USB A (the end that isn't micro) off the 15 foot cable. Cut the USB A male to female cable in half. Strip everything back to expose all the wires. Cut away the foil shielding but don't cut off the outer shield strands. The USB A female connects to the audio DAC. The USB A male connects to a small USB car charger. The micro USB (on the end of the 15 foot cable) connects to the back of the modified car dock.

Fortunately, USB wire color codes are standardized, so you can rely on the wire colors.

Cut off the white and green wires on the USB A male wire. Since the car charger only provides power, there is no need to hook up the data wires.

Connect the two green wires on the USB A female and the micro USB cable. Also connect the two white wires on the USB A female and the micro USB cable. That creates the USB data connection between the phone and the USB DAC.

Connect the three red wires together. Connect the three black wires together. This will send the power from the car charger to both the phone and the USB DAC.

Connect the three "wires" you created by twisting the shield wires together. I had a hard time getting it to take solder. I had to add flux paste and heat it up a lot.

Inspect your connections to make sure they look good.

At no point should you have wires of a different color connected together.

I finished up my cable by putting small diameter heat shrink tubing over each solder connection (5 in total if you count the shield). I put a wire tie around the three cables for strain relief. I finished with a piece of large diameter heat shrink tubing over the whole mess. It isn't pretty, but it works.

I ran the 15 foot micro cable under the dash and into the back of the car dock. 15 feet ended up much longer than I needed, so I coiled the extra in the center console. If you had more forethought, you might figure out the cable lengths you need and cut the wires to that length.

Overall I am very happy with the solution. I get an easy, single connection to the phone in a dock. I get good quality audio without any digital noise or other squeals. I do find it goes a bit wonky (like a skipping CD - remember those?) when I get an email notification. I'm still trying to find a solution to that one.

It should be noted that the cable must plug into the dock to work (if you plug the cable directly into the phone, you omit the grounding of pin 4 to put the phone into OTG mode).