The bulb holder should twist out from back of the lens to seperate the lens from the bulb holder and wiring. The bulb then just pulls out from the holder.
All the 01-07 impreza & 99-03 legacy should be the same as standard and they work as i described above. Unless a previous owner has put in some ones he bought off ebay or something maybe? You can buy standard replacements from ebay, that where i got mine but you will then be missing the bulb holder so may have to pick that up off a breaker.
As johnny says, stick up a pic and let us have a look