Prepping the STi steering wheel
Now we need to get the STi steering wheel ready. Presuming that the air bag is still attached, you will have to remove it in the same fashion as the WRX airbag. If the airbag is wired in, you will need to get a small flathead screwdriver and lift up the black clip on the airbag plug. Once the clip is lifted up, you can remove the plug from the airbag. If there is no plug on the back of the airbag then you need to stop reading this and get one. Once you have the plug, please continue with the install.
The horn is hooked to the frame on the back of the airbag, undo the spade and set the airbag aside. Once the airbag is off, you will need to remove the 5 screws that hold on the rear plastic cover. There is a crab claw clip at the bottom, spead the two tabs apart and the plastic cover should come off.
Next, you will need to remove the cruise control switch. All of the screws should be out, so it should come out easily.
Then you will need to remove the metal plate that is underneath it.
Comparison of STi parts to WRX parts
Here are some pictures of the STi cruise control parts and the WRX cruise control parts.