The bulb warning system is very sensitive to resistance, and therefore you must make sure that all bulbs are the correct wattage.
Typically the tail lights are the culprit. If you remove the rear covers (inside part) you will see the correct bulb wattage embossed near each of the bulbs. Should be either 10 or 22W I believe.
Also make sure all of the bulbs are Philips, Sylvania or Osram, with the nickel-plated sockets. Those cadmium-plated ones show a slightly different resistance and will set the warning lamp on.
Also, look for any bulbs that are discolored. A discolored bulb may still be operational, but may be on its way out, and will set the warning lamp on.
Also, with age and vibration, the taillight sockets may lose contact with ground and cause the lamp to go on. Jiggle each bulb and see if the lamp goes off. If it does, then adjust the socket prongs to allow the bulb to seat more firmly.
Hope these tips help.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car