The taillight/backup light/license plate light assembly is separate from the brake light/turn signal light assembly on the Buick LeSabre. No matter what rear bulb you have to replace, you must access the trunk area to do so. Although a modest investment, the amount of time and effort it takes to remove the assemblies, you should consider replacing all the lights once the unit is disassembled.
Open the trunk of the Buick LeSabre.
Locate 7 nuts underneath the trunk lid (3 on each end of the trunk lid and 1 in the middle). Remove each one using a nut driver.
Manipulate the trunk lid at chest level and gently pull the rear taillight assembly slightly away from the trunk lid.
Locate the burned out bulb sockets and then remove the bulbs. Insert the new bulbs into the socket and test by turning on the lights before reassembling.
Replace the tail light assembly by aligning the studs inside the predrilled holes in the trunk lid. Refasten the 7 nuts.
Remove the cargo net if applicable to replace the brake and/or turn signal light.
Unscrew the plastic wingnut that the cargo net is attached to. Pull back the trunk liner.
Remove the 2 plastic wingnuts under the trunk liner.
Pull the brake light assembly outward from the back of the LeSabre.
Locate the socket of the burned out bulb and then turn counter clockwise 1/4-inch to release it from the assembly.
Remove the old bulb from the socket and replace with a new one. Test the light before reassembling.
