Quite often Datsun switches can develop corrosion inside the switch over time so it's defintely worth dismantling it and using fine sandpaper to clean the terminals.
Please don't be offended by this question but have you put 12 volts directly to the two lights in question to make sure they haven't both blown in tandem?
Try that first and if that's fine pull the connector off your light switch and make sure you've got 12 volts at the switch. Then plug it back in and make sure you have 12 volts coming out when the lights are in the on position.