I just installed side markers on my 240. Run one wire from the marker to the wire that powers your front turn signal. The other wire from the marker goes to a ground connection. This way, the side marker is always off except when the turn signal is on.
I've read that it can also be done by connecting the wires from the side marker to the wires that power the parking light and the turn signal. This way the sidemarker is on when your headlights are on, but it flashes with the turn signal. It has to do with which of the two circuits (turn signal or parking light) that the sidemarker gets it's ground from. There's a diagram at Daniel Stern's lighting page, lighting.mbz.org
In case you're wondering, the two wires from the side marker are the same, so it doesn't matter which one gets 12V and which one gets grounded.