It should workon the horn lights as far as the flashing part.. You'd have to figure the "Constant on" part though. It shouldn't be too hard, just figure it by running a constant hot that would be switched by the ignition.. I also did a how to on adding an ignition switched fuseblock that would help out.
Here's a couple of links...

Switched Fuse Block

Flashing Side Marker

Hope it helps..