If I buy any after-market speakers for the speakers in the back of the car (rear deck?
) can I just hook them up with my existing wires and head unit or will i need to get new wires and a head unit to let my speakers work there best?

 

first off is it the mach system or is it the base system?

if its the base system you can just replace the speakers and be just fine. though changing the head unit would give you better sound.

if its the mach system and you want to keep it stock looking. finad a speaker that is more of a bass 6x8. all it needs to be is a 2 way, because all you are going to hear is the mid and lows. but even here changing the head unit will yeld the best sound.

 

the mach woofers are 8 ohm woofer so it is difficult to replace them with a 4 ohm full range speaker and it is not recomended.

As he said it would be better to start off replacing the deck.

 

o sorry its a 07 gt not a mach1 system shaker 500.. don't know how I forgot all that haha

 

with no shaker system then itll be fine and you can use whatever speakers that u like but defly look at getting a new deck that will make ur stock speakers even sound better