Dimmer switch location on 03 GMC Sonoma - 2003 GMC Sonoma

Consider the dimmer switch on the steering wheel; the wires from it wouldn't affect the tail lights, but could possibly affect the headlights. Hopefully loose wires from the assy is the problem.. replacing the unit might be difficult. or expensive.

That code is for your evap vent valve by your evap canister back by the tank I would check the connection on the vent valve and make sure its not corroded or unplugged there isn't a specific relay that controls it its part of a bigger circuit

Check power in and out of dimmer switch.

Is the dimmer turned to the light on position by chance

yes it is probably fuel pump which is located in the gas tank. gas keeps pump from overheating. never let fuel level get too low in tank...

have to get code from dealer for security purposes he gonna ask you the radio serial numbers or the vin number

03 sonoma will not go in 4hi but will go in 4lo

Did you try adjusting the dimmer switch for the dash lights?

a problem with your dimmer switch; may be able to adjust it,if not replace it but inspect wiring and connector first

These two links have the answer:
http://www.obd-codes.com/p0410
http://www.obd-codes.com/forums/viewtopic.php?f=1&t=1956

Good luck!