My 03 mustang gt has no headlights both high and low doesn't work the flash works though I checked the dimmer switch plug and it was cracked and had burn Mark's on it so I replaced it and the multi switch i haven't replaced the dimmer yet though
Before throwing more parts at it, are you sure you have power? Check the (BJB) engine left front near battery Fuse #2 (hot all the time) this feeds the Main Light switch and the Multi Function switch. Also check the (CJB) under dash left side #4, 10, 38 for power if the Main and Multi are working.