should fit no problem, as long as both are WRX. STis have a diffrent rear sway bar which i think you need new endlinks to use (?). is it a early model 03? the late model 03s have the same size (17mm) bar as the 04+. the 02s are guaranteed to have a larger bar (20mm).
the only thing i had to do is grind down the bushing that goes around the bar and to the bracket on the mount. the newer models have a slightly different metal bracket that goes into the sway bar mount.