I just got done replacing my stock speakers with Infinity Kappa's in all 4 doors. I was able to put 6.5 seperates in the front and 5.25 in the rear all this with the assistance of Innovative Automotives speaker adapters (thin). the Infinity Kappa 6.5 are a perfect fit in the front doors as for installing the Kappa 5.25 in the rear doors prepare to do some minor trimming. As for the wire polarity it is listed below:

Driver Front
Brown with white trace= postitive
Green= negative

Driver Rear and Passenger Rear
Blue with yellow trace= positive
Red with white trace= negative

Passenger Front
Red with yellow trace= positive
White with black trace= negative

again this is for the 05 Sti and I cannot not speak for the WRX or 2.5RS.

 

Are you talking about having to modify the speaker mounting location in the panel?