I think your talking about the bulb out indicator light, correct?

If so they are fairly sensitive and compare the current draw for each bulb.
The indicator light can come on because of added resistance from things like dirt or corrosion in the bulb sockets or mis-matched bulbs.

First thing to do is pull the bulb sockets out and clean the contacts up, a pencil eraser works well. Also clean up the bulb bases and the contact on the tailight housing. If you still have the problem try new bulbs, both sides out of the same package.