2003 Mercedes Convertible Door sensor not functioning properly

There is a switch in the trunk lid of my 2001 SL that tells my car that the trunk is open or closed. Your 2003 probably has a similar switch. If that switch is misadjusted or defective it may be the source of your incorrect indication. Sit in your car with the ignition "on" and have a friend push upward on the switch as far as it will go. If your false indication clears-up the switch needs to be adjusted. If it does not clear-up I think that either the switch is bad or you have a wiring problem.