Beemer is also the standard slang for a BMW in Ireland. And Merc (pronounced Merk) for a Mercedes-Benz, Jag for a Jaguar, Rolls for a Rolls-Royce. It only seems to be the luxury brands that get these nicknames. But sorry, Lexus - you're just called Lexus!
In India, a Merc was the hip (for young boys) name for a Mercury, as in Ford.
I enjoy these 'debates'; at least nobody tries to frame rules for their application.