The speakers and wires can definitely stay. The main thing you have to be concerned about is whether there is a separate amp in the car, because you might need to wire around it. As you don't have the premium sound system from the factory in this car, there probably isn't an amp, besides what was in the original head unit. How many speakers are there around the cabin? If it's just six or eight, then there probably isn't a separate amp. If there is a subwoofer (check the trunk), then there probably is an amp.
I recently purchased a Pioneer DEH5000UB for my car and was wondering if there are any installation concerns that I should be aware of. 2002 Lincoln LS without the alpine system. I am asking because one of my buddies said that I need to also change out my factory speakers and speaker wires because he believes that the wires are amplified or something of that sort. Any response would be greatly appreciated.

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