many times, in cars where a stock amplifier is present the entire system has been designed around that concept from the factory.
These factory amplifiers have built in XOs and sometimes the speakers they utilize have strange ohm configurations...(like 3ohms for instance).
your best bet is to either A) see if there's an adapter available for this specific make, model, and stereo option or B) pull the factory amplifier and speakers and replace it all.
If you go with B you can either power the speakers off of the aftermarket head unit or power them off of an aftermarket amplifier...depends on how much money you want to spend. However, seeing as how you have opted for Pyle speakers, I'm guessing that you're wanting to spend as little as possible.