I had the light bulb problem too, and even after switching out rear taillight bulbs to the "correct" type and wattage, I accidentally discovered that one of the metal ground rails that hold several of the bulbs in their sockets was slightly "kinked", and would interrupt voltage enough to cause the indicator light to come on.

I was able to bend it enough to cure the problem, but after backing into a tall pickup truck and cracking the taillight lens, I decided to get a new housing to completely erase the problem.

BTW, the warning light would only come on during the day when I didn't turn on the headlights.

I also know of another car where replacing the headlight switch cured the problem. (I tried the same before I found out about the rail, and of course, it didn't work for me).

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car