I Used To Work In Car Audio Install For Over 15 Years On Both The Retail And Dealership Side Of The Business.i Found A Solution To Your Problem Years Ago In The 1990's.you Can Add Any Din Sized After Market Head-unit To Any Factory Amplified System Using One Of Two Methods Listed Below:

1.connect Rca Cables To The Rca Output At The Back Of Your Alpine.cut The Cable In Half And Use The Covered Wire As Your Positive Input And The Braided Surround Wire As Your Negative Input Into An Factory Adapter Harness That You Should Be Able To Find At Any Car Audio Retailer. I Have Upgraded My 2005 Roadster Keeping The Factory Amp And Speakers With A Sony Head-unit With I-pod Connectivity,hd Radio, Satellite Radio, And Cd Changer Controls.i Also Added Two Amps Component Speakers In The Dash And Kickpanels.i Am Working On A Solution To Add A Speaker Pod Between The Seats For Rear Imaging. Let Me Know If You Have Any More Questions.
I'm Sorry I Forgot To Mention The Other Option Which Is The Best And Allows You More Flexibility. You Can Use Pac Speaker Level Converters Instead Of The Rca Outputs On The Alpine.the Converters Are Adjustable And You Can Set Them To Really Make You System Sound Great.

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