Depending on the type and strength of the sealant GM used at the factory; you can heat up the part in an oven say 200*F for 3-5 mins should make the sealant soft enough to pull the lens from the housing. Checking often you dont want to burn the plastics, just warm the "glue".
This method worked for me back when I used to do retro-fitting projectors into headlight housings on Lincoln LS's, till Ford used better sealant on the later models.
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