2 inputs same speaker

I Have a brand new 2008 Mitsubishi Lancer that I'm going to put a high end stereo in.  Problem is these new cars aren't meant to have the factory deck removed, so i'm building an automotive pc to power the system.  I'm just going to cut the speaker wires and run the inputs through amps, leaving the outputs from the factory deck unconnected.  However my car has a bluetooth phone system built in that I would still like to use.  I'm worried if i just hook both the deck and amp's outputs to the same speakers, it would send power back through my factory stereo (and probably my cars computer) and cause major damage, being that the amp will be pushing far more power.  Is it possible to hook up the ouputs of my deck through speaker level inputs together with rca inputs on the amp to get both signals working together?  Or maybe using diodes in order to prevent damage.  I would appreciate any help or ideas. Thanks.

I never heard of the cleansweep before.  I like the idea and might get one, it is perfect for what i want to do, but it seems there might be a cheaper method.  If all else fails i suppose i could just use a switch/relay to switch between functions.