Just wondering what size the rear speakers are in a wagon, are they the same as the fronts 6.5"?? Currently have 605cs in the front un-amp'd and wanting to improve the sound at the rear. Been looking at the Infinity 625s that have the tweeter built in, any thoughts.

I know I need to amp the fronts, should I do this before the rears and then forget about upgrading them??

Thanks in advance

Best to upgrade them for a set of infinity 452i or similar.
If your running a sub, power the fronts and sub using an external amp/amps and the rears from the h/u.
If your not running a sub, it would be best to run front and rears using a 4 channel amp. Run the fronts full range and use the amps high pass filter to filter out the bass (approx 120hz and under) from the rears.

Oh almost forgot. The rear door speakers in a y2k impreza wagon are 10cm coaxial.