A number of problems can cause this, but typically, the source is somewhere inside the taillight housings...and not the tinted lenses.
Make sure bulb wattage is correct...the proper bulb wattage is embossed on the taillight housing next to each socket.
Make sure the bulbs are the nickel-plated OSRAM Philips or Sylvania bulbs. The gold-toned cadmium bulbs send a different resistance and will set off the warning lamp.
It's fairly simple to rule out particular bulbs if it comes on say, whenever headlights are switched on or the brakes are depressed. But a "constant on" situation usually indicates a ground (or earth) problem.
The taillight housings each have two metal rails that house the bulb sockets. Over time, they loosen from vibration and lose contact with ground. When this happens, some of the bulbs may not illuminate properly.
It's not obvious by looking, but the total resistance seen by the warning system as a result of the improper gound is enough to set the warning lamp on.
To verify this, take hold of the taillight housing and press firmly on on one of the metal rails with your thumb (primarily the ones with the most bulbs). If any of the bulbs suddenly get brighter, you've found your problem. Press down onto the rail until it "clicks" back into place.
The warning light won't shut off until you turn the key to the off position and turn it back on again.
Hope this fixes your problem...
