Prefacing with I have not done this myself or committed to any extensive research yet for our GX, but I think the 4Runner uses the same override as the GX. I'm not 100% unfortunately. A few GX guys are running the beatsonic here Beat-Sonic TVK66EP Navigation and DVD Control Bypass Module Combo
Another resource to check out is the GXOR fb group, there are several that have done some in depth mods to the head unit that may get you in the right direction.
Try ih8mud for reference too. Several of the 150 and 200 guys have similar information for the beatsonic override as listed above.