I've searched the forums and I'm not sure this particular question has been answered. I bought another 2008 Prius to replace my previous one that was rear-ended and totaled, and the "new" Prius had a missing rear wiper arm. No problem I thought, I'll just buy a replacement from Amazon. 1) The problem is that after I remove the nut, I'm having trouble removing the plastic piece. It seems like it's attached to a rubber gasket underneath that stretches each time I pull on it. 2) The replacement arm I bought has a rounded center and I was under the impression there were teeth involved.
Went through this not too long ago. I believe I just pulled up on the plastic part and gently tapped the threaded portion with a brass hammer (preferably with the nut on but loose, so you're hitting the nut rather than the stud). There is a spline on the wiper arm shaft that will dig into that soft metal when you tighten the nut.
If it is anything like my Volvo, the arm is aluminium and the rest iron. So it will eat into eachother and 'rot' or 'oxidise' solid. Horrendous to get loose and since you're working near the glass window, incredibly delicate work. (General info, I'm not sure the OP will have this problem). I tapped it with a pin (the white stuff that forms between the aluminium and the iron/steel), sprayed it with WD40, wiggled it numerous times, etc. and eventually it came loose.
Anyone have any idea of the tightening torque spec for the nut after this rear wiper arm is replaced?