Restoring SMS and YM2413 functionality on Genesis 3 VA1?

Came across this thread showing a simple mod to restore SMS compatibility with the Genesis 3 (VA1).
http://www.tototek.com/phpBB2/viewtopic.php?t=4371&sid=389044064ccfdd6080702a13947e26d8

EDIT: just did the the mod (along with a region change to Japan), and it doesn't appear to enable the FM synth with the Powerbase FM. Looks like there are 2 wires that need to be connected from B1 and B3 on the cart port to "Audio Circuit" as mentioned here: http://www.sega-16.com/forum/showthread.php?10676-Genesis-3-Compatibility-Restoration

Any idea what lines on the motherboard to connect to for the pass-through audio?