You're right to not believe them. There is a specific service action for
squeaking hoods and windows where the dealer cleans the seals and glass then
applys a lubrcant called "Gummi Fledge" (sp). Tell them to look on their TIS
and they'll find it. Its worth buying a tube of the stuff yourself from them
just to get rid of the squeaks as they appear, only a few ?/$

See if the dealers parts department still has any BMW "Gummiphledge."
This is a silicone grease in an easy to use applicator.

Put the top partway down - and use the Gummiphledge on all rubber seals
and parts. Leave a very thin coating on any surfaces that contact other
surfaces.

Your noises will now go away. My '98 M3C was horrible when I got it -
and absolutely quiet now.

I understand Gummiphledge has been discontinued by BMW, but I was able
to find it from another source (actually in the manufacturer's tube)
after doing a web-search on it.

HTH