Yep, odds on it's actual headlight switch in the dash. Nissan decided to have the lights draw all the current through this switch, instead of run a relay. So over time the plastic contact lugs melt.
When my car did this, I just got a 2nd hand switch which fixed it. Plan to wire up the relays when i've got some spare time.
If you wanna have a go, this should help: http://www.danielsternlighting.com/tech/relays/relays.html