Car Stereos: 6x9 wiring, factory speakers, toyota celica

Question
Hi, just a simple question for you here (i know zero about speakers and systems). I have a Toyota Celica which has front and rear speakers as standard, and i have a pair of 6x9 speakers that i want to install into my parcel shelf. All i want to know is this: is it necessary for me to faff about running the wires through my car (under carpets and all that) in order to connect them to the head unit, or can i just somehow run them off the existing rear speakers?
Thanks in advance, Carl.

Answer
Hi Carl,

There's actually more to deal with here.

Your factory speakers likely have an impedance of 4ohms.  Your new speakers also have an impedance of 4ohms.

If you wire them in parallel, this will bring the circut down to 2ohms each, which will blow your deck.

Your options are to run them in series, which will increase the circuit to 8ohms, and will probably be QUIETER even with new speakers, disconnect the rear door speakers, or add an amplifier.

Justin