Yeah, that's a new one on me too. Where I'm from, "Merc" is usually gearhead speak for a Mercury not a Mercedes. Then again, I'm old and out of it, so what do I know.
Mercedes-Benz, a motor manufacturer and its products (primarily U.K. slang)
In the last 10 years I don't think I've ever heard it referring to a Mercury, then again nobody talks about Mercury anymore outside of maybe the Marauder once in a blue moon.