I tend to agree with you, Norm. The Volvo direction pamphlet shows a tube of adhesive which is marked "Silikon lim" (silicone glue?)on the cover. Also, the text of the directions mentions that the stuff emits a "vinegar acid" while hardening, and I've always thought that silicone sealant smells kind of vinegary before it cures.
I'm going to try using Goop(tm), which is a contact adhesive and sealant that I have been using in other applications with good results. The stuff is clear and fairly strong. I'll let it set up for 24 hours while the assembly is installed on the car, but with the backup lens taped down tightly with clear packing tape. That's not what the directions advise, but I need the car and haven't got a spare lens assembly, so...