Genuine SEGA Mega Drive II RF Switch MK-1633-04 Aerial Splitter Box | eBay
Just completed an S-video and stereo output mod on this non TMSS Model 1 Genesis. No holes were drilled in the case, used existing RF modulator holes. Before and after pic. :
Retro-Bit RF Unit Adapter For Sega Genesis 2 system and Game Gear - Walmart.com