they actually told you that you can't use the top, and the problem is with the manufacturer of the top motor?
I'd tell them that (1) it is not your concern whose fault it is (2) that you don't care whose fault it is (3) that they had better fix it (4) and that you paid good money for a convertible.
Then I'd let them know since they are forcing you to now have a coupe (rather than a convertible--no putting down the top), that you'd like to talk to them about refunding some of the money you paid for the car--like every cent over the cost of a comparable coupe.
It's drastic, but the dealer sounds like they are jsut taking the easy way out. One motor going bad can happen...but the new one going bad right away sounds like an installation screw-up to me, and now they are just covering their bases.
I haven't seen any mentions of top problems on any board. The problems with the 2005's (gas tank, strut popping, radio skipping) are all very well known here, and were known long before most of us even got their cars!! I think if it was going to be a recall, we'd see a lot more talking about it here.
I smell dealer BS. Find a new dealer.