Is there power getting to the switch? If so you could make a jumper wire with male spade terminals on each end & jump the IN & OUT on the switches receptacle to bypass the dimmer to see if the lights are working, if they do then you will know the switch is the problem. (contact cleaner may help or see member SLOTTS for a rebuild) If the lights do not come on then you will need to trace where the problem is, wiring, ground, bad connection.
As to your question you can isolate the light bar, or any accessory for that matter, with a jumper from its plug to any 12V power source to check.