I recently purchased a new head unit for my 08 H3 and after some research noticed I may need to buy some kind of special adapter for everything to work. Is this true or is it optional? If so where is the best place (cheapest) to get one?

 

Do you have the Monsoon system?
Sub-woofer in the rear driver side behind the panel?

Don't buy a cheap universal one, you will have nothing but problems with it and have to splice wires etc.

 

No mine is the base model. But I pay monthly for onstar so i need to keep functions.

 

Go to crutchfield.com, research what they have, price it on the web, buy it wherever you want.

 

Metra GMOS-01

or

PAC RP5-GM11

Either will keep your Onstar and Chimes.

 

both will retain onstar and warning chimes?

 

^^^^^This, I had a Buddy that did Exactly that & definitely had nothing but Problems!