Since Subaru wants you to just buy the Sti, they do not make this kit specially for the 04-05 wrx you have to do some cutting and modifying to your car, and the corner splitters.There for I will not be held responsible for any mistakes. This article should be used as a reference. Always measure twice. If you don't have the basic skills to do the install do not attempt to drill into your brand new car!!!!!!! !Cry!
If you have not purchased the kit yet you can find it at www.subaruwrxparts.com
for about $139 including shipping.
When the kit comes in it should contain two boxes. One box just contains either the right or left splitter. The other contains a splitter, and a bag of six screws and six clamps.
First locate the plastic pins on the front bumper and pull them out by using a small flat head screwdriver there should be three. One at the wheel well, one about 5 in. away from that under the bumper, and one about 7 in. from that also under the bumper. Do this on both sides.
Second test fit the corner splitter(s) by holding it up to area
Third look on the back side of the corner splitter(s) and find the white pins. Remove the pins and use a dremmel tool or knife. to remove the brackets that hold those pins in. be careful not to cut through the splitter(s).
Fourth you must hold the splitter(s) up next to your car again. Be sure it i tight fitting and make markings in the groove of the bumper where the 3 flat inserts on top of the splitter(s) touch.
Fifth using a dremmel tool cut slots for the flat inserts where you made the markings. Be extreamly carefull not to cut the slots to wide or long or the splitter(s) may not be as secure.
Sixth fit the corner splitter(s) to the bumper and reinstall the pins you pulled out. After you have put the pins in use the supplied screws and clamps and find the mounting holes for them they are very noticable the first one is the first hole you come to in the wheel well, the second is the first tab under the bumper right beside the wheel well, and the third is inbetween the 2nd and 3rd pin that you took out and reinstalled.
finnally take the screws and clamps and install them in the mounting holes described above. Be sure the screws are secure and tightened properly.(Hint: Using 2 flat head screw drivers insert them into the clamps and pry the clamps apart a little for easier install of the clamps and screws.)
Hopfully this will help you with your installation. I just installed this kit along with side strakes on my 05 wrx. Just remember to measure and remeasure, and it should look great when installed just like the Sti.