2003 chevy trail blazer lt hack job

One of my good friends took his truck to a local shop to have his head unit installed and they really hacked it up. Some how they fried the factory head unit. Well this shop is out of business now and I am stuck with the task of "fixing" it lol.  He has the aftermarket unit in and it works but not correctly. He has a door chime so i think they did end up finally getting the correct harness. He told me they charged him 130.00 for the harness, so i think the correct schosh harness was used. Anyways his onstar doesnt work and he also wants to use his steering wheel controls. I have been out of the install game for some time now and im not sure what moduals to use for the newer cars. I know pac makes some and schosh makes some aswell. What Im asking for is a part number for the right unit for his truck, and what to do about the onstar? I have done alot of research on this and searched on here a lil and this seems to be a "can of worms" but i still need to do this for this friend so any help would be great ~!

Thanks in advance

Branden Jones

Im an idiot.  not sure about the scosche kits, but metra has a few different ones but i think GM-RC01 is the correct one.  Pac audio does make a universal steering wheel control kit, i think its the SWI-X

You  need the OS-2BOSE from PAC to retain the onstat and bose or if it just has onstar the OS-2 will work.  The sterring wheel interface will depend on what radio you use.  SWI-X, universal

SWI-ALP, Alpine, Kenwood, JVC

SWI-PS, Pioneer  Sony

etc