Toyota '94 Cover of high-mounted stop light won't come off

My '94 Toyota Camry sedan has a burned out high-mounted, rear-window stop light. In instructions to change the bulb, my owners' manual shows a diagram of the covered high-mounted light with an arrow pointing toward the front of the car. No words. I have tried sliding the cover off and failed to budge it. I cannot fit my fingers between the light and the rear window to get leverage. Gingerly, I tried prying off the cover but felt little give. I stopped because I feared cracking the cover. Can anyone rescue me from my feelings of being an idiot? Thanks for trying.