I have pictures! Will a supported member help me post them please?
This is a How-To make your own Aux Input jack kit like the OEM part no. H6210FG000 for a 2007-2008 Impreza without Premium sound. I do not know what else it will work for, but if the 8-pin port in the back of the stock headunit looks like the one in the pictures, then it should work.
The MSRP for the OEM kit is $75 (you can find it easily for $62). This includes a module housing a 3.5mm TRS female connector for a headphone-style jack and a specialty cable that connects the module to the back of the factory headunit. It is of note that this $75 kit does not include the necessary TRS cable needed to play any audio device such as an MP3 player.
H6210FG000 Installation Instructions
I don't think I'm alone on this when I say this is a GIANT rip-off. This thread is a little F-you to Subaru.
I spent $9.50 at Radioshack for the 2 parts, and since one would have to buy the $7.50 cable anyway, I duplicated the OEM kit for $1.99. The car is leased, so the setup is super easy to remove and does not leave a trace of tamperage.
Refer to the "Real" Aux Input on 2007 OEM Radios section of
Stock head unit information -- pin-outs, aux inputs, and more
I have confirmed the pin assignments are identical for the 2008; Pin-1 is left stereo, Pin-2 is right stereo, Pin-5 is audio ground, and Pin-6 is a special Aux enable switch.
When looking at the back of your headunit, with the 8-pin connection port clip recess pointing upwards, the top row, left to right goes Pin-4, 3, 2, 1. The second row, left to right goes Pin-8, 7, 6, 5.
You will notice that when you press the CD/Aux button on your radio as it is, it will not go into the Aux mode. There is a +5-volt source at Pin-6 and when you ground it, it enables you to enter the Aux mode. I'm assuming the OEM kit has a switch in the female jack that senses when you plug a wire in and then enables the Aux mode. Since disabling the Aux mode is practically a useless feature, I will permanently enable the Aux mode by grounding Pin-6.
1) The hardest part is finding suitable connectors to attach intermediate wires to the pins. The connectors need to have an internal diameter small enough to fit snugly on the tiny pins, have an external diameter small enough so you can fit all four connectors on all four adjacent pins, and need to be completely insulated all around.
I found these tiny connectors "Telephone Butt Connectors" at Radioshack for $1.99. If you have scrap wire laying around, then you have successfully saved at least $60. Celebrate. I cut the extra plastic off one end of these connectors so that the internal metal sleeves would slide all the way onto the headunit pins. The inside of the connectors were still too loose for the skinny pins, so I hooked the intermediate wires under the bottom of the connectors so the wire and the pin would both fit very snugly inside the metal sleeve.
2) I used a 6-foot cable that had a 3.5mm TRS connector (standard headphone jack) on each end. I lopped off one of the jacks to expose the 3 wires. The uninsulated wire is the audio ground. I don't think there is a standard with the wire coloring (red, black, sometimes white), I've seen them reversed on cables, so it's best to use a ohm-meter to determine which is left stereo and which is right stereo. The tip of the jack is left stereo, the ring (middle section) is right audio, and the sleeve is ground.
3) Now that you have a solid connection with the pins and 4 intermediate wires are sticking out of the back of your headunit, you can connect the stripped end of the TRS cable to the appropriate terminals. Attach the 4th Enable wire to one of the Phillips screws on the headunit brackets, similar to the instructions for the OEM kit.
4) Test everything before you start putting things back, then route the cable under the console just as the OEM kit instructions have you do. Plug in your MP3 player, laugh at the fools who bought the OEM version, and celebrate again.