This varies with model year.
The trunk
Bulbs or turn signal flasher, maybe even the switch.
how do you change a rear turn signal bulb on a 2010 buick lucerne
it's normally under the dash.. turn on the turn signal and find the device that is clicking. That shoudl be the flasher unit/regulator/relay
More than likely the Flasher Relay is bad.
replace the combination switch on the turn signal stalk
Sounds like you need a new flasher.
Turn signal flasher may be defective
Double-check me on this that the rear turn signal uses the brake bulb. That bulb is a 3057. See sources and related links below for more info.
The turn signal flasher is under the instrument panel to the left of the sterring colunm brace. (note I'm still looking for mine).
how to change front turn single bulb on a 2006 buik terriza
lever pulled out, having trouble getting the lever locked back in place