I have a 1995 Mercury Grand Marquis and my mother has a 2003 and they both are the same, so yours probably is also. After you pull the trunk lining away so that you can see the metal, there are 3 large black plastic wing nuts that you unscrew. Once you remove those, the tail light section will come out from the front and then you can get to the bulbs to replace them.
Why ? Just replace it.
check the bottom of the steering column
where dose daytime running lamp module go on92 mercury grand marquis need to replace one
Remove the wing nuts from inside the trunk, the assembly will pop out.
I have a 1998 ford grand Marquis- and the problem is the air suspension light stays on- how to fix this problem
check your brake lights if they not worhing replace fuse or brake light switch
Have the codes read.
Fuse, loose connection, or the light switch. how do you remove the dashboard on a 94 mercury grand marque?
You won't know until you get the code read.
where is the running light sensor on a 2003 grand marque
assuming you mean the light bulbs, see the answer to "how do you change turn signal lamps both front and rear on a 96 mercury grand marquis" right here in WikiAnswers.
Have vehicle scanned to determine the problem