@Super-Cuda,
@Kendog 170 and
@mvh are right. You should remove the interior trim.
I have replaced 67-69 Barracuda Fastback rear glass before, and it is not difficult. Although you may be able to remove and replace the rear glass without pulling the interior trim, removing the trim is not at all difficult. Also, remember why you are pulling the window out. You are looking to fix a leak, NOT just replace the glass. So you will want the interior trim out of the way to look for the leak source. You will need to pull the stainless trim around the outside of the glass. Then You pull the glass out. It is VERY difficult to 'pop' that glass out and save the gasket without damaging the glass. I personally would not try to save the gasket at the expense of possibly cracking the glass. I would, 1) remove all interior trim, 2) Remove the exterior stainless trim and then use a hose to try and find the source of the leak. If you find it, great. If not, I believe you are still looking at replacing the gasket while reinstalling the glass, plus doing some sealing during the process. Then I would 3) Cut the gasket away with a sharp utility knife, 4) Pop out the glass from the inside, 5) reinstall the glass with a new gasket and using a little sealant during the install. Apple Auto Glass can recommend the type of sealant and the proper application. I have personally never used sealant, but you could use some around the metal lip.