Here a tutorial about how to install a rear sway bar on a 2008+
I used a sway bar from a 2011 WRX which is 16mm thickness. You can use one from a 2008 to 2011.
2008 -> 15mm
2009-2011 -> 16mm
First lets start with the hardware that you need. Go buy them at a dealer:
2x Bracket -> 20466FG000
4x Bracket bolt -> 010108200
2x Bushing -> 20464FG000
2x Endlink -> 20470FG000
2x Endlink Nut -> 902370046
2x Bottom Bolt for Endlink -> 901000221 & Nut -> 902350001 (or you can use a M10x90mm or 3/8- 3"1/2 with the nut)
You can use washer on the bolt if you want like i did
Once you have all of the hardware needed,
1. Grease the bushing and the bar (they goes outside the metal sleeve)
2. Jack the rear of the car and secure it with Jack stand.
3. Bolt the endlink on the control arm
4. Slide the bar under it. The small curve on the bar goes upward and the bushing goes in the rear of the subframe bracket (You should see it). On a sedan with double exhaust it may be a little harder to slide it in.
5. Install the bracket and tight them in place
6. Bolt the endlink to the sway bar (Do not tight the bolt)
7. Put the car on the ground
8. Tight the endlink nut with wrench and Hex key
That how it should look like
And go for a ride!
For me its day and night, there is no more rebound from the rear suspension and its more stiffer! All the hardware (Except the bar) cost me around 30$.