What would be the easiest way to add a sub woofer. The car has the standard Swing and I assume it doesn't have a sub out? I? I'd like to keep the functionality I have with the Swing, eg MDI connection and bluetooth intergration. What would you guys suggest?

A high to low converter to an active sub. or a sub with high level in, is probably the simplest way but may not be the 'best'

Personally, I'd go for a decent sub and separate amp.