First I ask this because there are some (or perhaps someone knows of some) that decide to try regardless of what Remco or the manufacturer recommends. I may be one of those in the near future.
The 2004 (and earlier) Mustangs with manuals can be towed. The 2005 was a new body style, but with at least the same engine choices. If the transmissions didn't change, then I'm of the opinion they can be towed as well. I know the risk. No warrantee to worry about.
And Y-Guy, I understand your thoughts. But I used to have a 2003 Mazda 6 with a 5sp we bought just to pull behind a motorhome if (at that time) we were to buy. When that time came, Remco and Mazda told us this car could not be towed. And nobody made a baseplate for it. Some in the RV world disregarded this and towed them anyway. Now Remco states it can be towed. The same thing happened with the Mazda Miata. At first no one made baseplates as Mazda said they couldn't be towed by an RV. Now at least Roadmaster makes baseplates for Miatas and they have to be a great toad! I think with most manufacturers, it's more of a warranty issue than anything. RVers aren't a big market.