Ok, from what I understand skimming over whats written here, you have a Neo Geo Stereo RGB scart cable, which is wired up to tap both right and left audio from the A/V port.
Firstly, can I ask, does this cable actually fit into the Megadrive 1 A/V port? The Neo Geo AES and CD consoles take a "c" style 8 pin din connector, whereas the Megadrive 1 uses a "U" style 8 pin din connector i.e. it is of the offset variety where the top two pins are offset by a small number of degrees - I feel that your cable may not fit into the Megadrive 1, unless when you had the mod done they replaced the whole A/V socket?
The best way would be to purchase a new cable (I could supply it..) or construct your own. You can tap the right and left audio signals by soldering a two core piece of audio cable to a 3.5mm audio jack, then the other end of this wire goes inside your 8 pin din plug, and right and left audio are then soldered to wires which go down the cable to the relevant scart pins (2 and 6) - this cable then plugs into the headphone port of the Megadrive 1 therefore giving you stereo sound.
If however you wanted to go ahead with your original plans of routing the stereo signals to the A/V socket. I would go about it by doing the following: a piece of two core audio cable, inside the system, soldered to the right audio and left audio at the points on the motherboard where the 3.5mm headphone receptacle is soldered, then as per the above diagram use a multimeter to locate the relevant tracks on the motherboard behind the DIN socket for the mono audio and the sync* and cut these tracks just before they reach the socket pins.
* (Check first whether your Neo Geo scart cable syncs on "true sync" or composite video first, since either can normally be used in a scart cable)
Then simply scratch away some of the solder resist on the board after the points where you cut, and then wire right audio to one of the pins and left audio to the other.
Of course all of the above depends on the pinout of the 8 pin din plug on your cable, check all the connections first and make sure everything is correct!
It sounds like a hell of a lot of hassle to me though for the sake of an extra cable.
Also keep in mind, an unmodified Neo Geo AES outputs a varying level of RGB, i.e. early units supposedly output a better RGB than that of later units. The Megadrive 1 outputs a very nice RGB signal, but it is too strong, resistors are required on the R, G and B lines, of a value of 75 ohms each, without these the picture is overly bright and text is difficult to read because it blurs.
It may be that the scart cable you have now has no resistors on the R, G and B lines, there may be extra resistors added internally to the Neo Geo AES, so therefore if you do carry out what you originally intended, its possible you could get an overly bright picture which isn't nearly as nice as with the resistors..
Of course this just depends on the construction of your cable etc, and without looking at it I couldn't make any definite assumptions.
Good luck!
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Guess my post was completely and utterly wasted then.. good luck with the cable, you do know its a poorly-spliced together piece of crap, made out of cutting the end off some sort of el-cheapo PS2 scart cable.
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