What would cause the 'rear light failure warning' light to stay on in a 1996 Lexus ES300 even though no bulbs are burned out?

It may caused by a light installed that don't match the factory specifications, could be the wattage is a bit off or something of that nature. It could also be the lamp failure sensor. On the LS400 it is located in the trunk on the drivers-side just behind the backseat. The sensor activates the warning light when there is a break in the circuit for the brake lights or the tail-lights.