"Bluestar by Costartech"
These are the ones I know of. They are all shady at best and all require that you have to get rid of onstar. They basically unplug the onstar box and this plugs in to the harness. Are you making a new system that could use the built in mic (the one in the headline/dome light panel) and integrate to the radio WITHOUT getting rid of onstar? Good luck!
White Fender Hashes from Big Worm Graphix
Custom White Bowties
GM Factory Remote Start and Factory Key Fob
Factory RS HIDs
Plasma P13W fog lights
Non-RS to RS and fog light harness by Gen5DIY
LED Interior, Trunk, and Licence Plate lights in white
ZL1 Spoiler with Gen5DIY harness
3 inch round cut, rolled exhaust tips
Blacked out tail light bezels