Help identifying gasket for replacement on SUT

[glennf1]

Hello,

Going through a carwash with my SUT and my gasket ripped off of the rear marker light base moulding. I have identified the part which is #2:

http://www.wholesalegmpartsonline.co...modelYear=2005

and sits on top of #38:

http://www.wholesalegmpartsonline.co...modelYear=2005

All I need is the small rubber gasket that sits between the two. Does the whole cap need to be purchased or is this standard GM gasket that can be purchased by itself? All the description says is "includes seal" which is just the rubber gasket grooved on both sides.