Simple-wring problem, or you just need to replace the bulbs.
Assuming the rear signal lights do work, check the front bulbs first.
Defintely a fuse or a bulb, check them all.
Blown bulbs.
Start by checkin all fuses and then go through all your wirein to rear lights and the check build recepticals for wear and corosion..
If the brake lights on a vehicle do not work on one side, this could be an indication of a blown bulb. Also, some vehicles have front lights that affect the rear lights, so checking for problems in the front lights could help as well.
make and model year please also do the front park light work or are you missing front and rear?
The multifunction switch inside the steering column is bad.
it is most likely a fuse or at worst the turn signals and stop lights work through the turnsignal switch
Try replacing the headlight switch. Do the front parklights work and not the rear ones? If so check the harness to the rear of the vehicle for breakage.
There are separate expansion valves in the rear, so even if the front system is failing, the rear unit could still work normally.
pull the light swithch once for front twice for rear that puts both on
Check your fuses.