I was told that I need an specific adapter to have Onstar work if I change you the head unit. I dont think he is talking about the PAC adapter. Has anyone heard of this?

i believe you do have to have an adapter, also allows your door chime to work.

Soundgate makes all the adapters that you need. if you can not find a dealer near you i can supply them. we are a car audio store.

The metra adapter works for onstar trucks too.