At 2220 Pacific time, a Cessna 172N taxied into a concrete drainage ditch while attempting to enter the taxiway at the North Las Vegas Airport. The pilot and passenger were not injured. The pilot said they had flown toward Red Rocks to watch the sunset, then returned to the airport. After landing he taxied clear of runway 7, thinking he was at taxiway C, but taxied into the drainage area. The pilot said he knew the landing light didnt work before he departed on the flight.