I ran side markers for visibility and safety, screw inspection... Flush mounted tail lights give you no side profile at all.
I just grounded mine through the bright circuit, so it flashed with the turn signal with lights off, and against the turn signal with lights on. Made it go out with brakes applied and lights on, but I was happy enough since I didn't have a just-signal light to work with back there on mine (though you do - you could put a light between the tail light and the signal. If it's an LED you need a rectifier since polarity will run both ways)