What I meant to say was, The nut on the back needs to be held in place with another wrench while turning the bolt on the front. On my car those rear nuts are welded to the back of the mounting plate. I was thinking that they might not be on your car.
Also the wire connecter plug that attatches to the valve has a spring lock clip to keep it in place, a thin sort of U-shaped clip. To remove the connecter press the clip toward the connecter on the wide side where there is some space between the clip and the connecter, like you are trying to slide it on further. This moves the two short sides of the clip out of their locked position and then gently pull the connecter away from the valve. To replace the wire connecter just push it on until you hear the sides of the clip "click" into place.