Yes, there's a rubber cover on the left hand side, under which is the 3.5mm stereo audio jack and an MCX jack for an external GPS antenna.
However, you can only attach an antenna when the unit isn't fitted into the motorcycle mount as the hard buttons down the left side of the mount obscure both jacks. Having had a Nav IV for a few years now, I can't think that I've ever needed to fit an external antenna to it, even though I do have several of them.