2002 cadillac deville radio

I'm installing a aftermarket hu into a 02 Deville. A big problem im having is when i unplug the oem and install the aftermarket none of the dash reading are showing up, and im getting all the troublecodes (low tires,check battery). Along with that problem I need to bypass the factory amp. What can i do to fix the first problem,is there a bypass kit for this? I was thinking of leaving the oem installed and tap into the wires I need then hook them up to the aftremarket. I would just need to tuck the oem in glovebox. Can anyone can please provide me some info?

What harness did you use...?M.E.C.P & First-Class

Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!