"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 with Custom White Bowties
GM Factory Remote Start and Factory Key Fob
SS Fascia, 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
Factory NPP cat back
Blacked out tail light bezels
Factory 20 inch wheels wrapped in Nitto Motivos
Lowered 1.25inch on Eibach Pro springs