Only One Brake Light Works - What's Wrong? Think it's left-turn signal?

[Hack&Lube]

Dad just called me, his old 1997 Oldsmobile Achieva has one tail-light out. The problem is that the bulbs are fine. The lights light up on both sides when you start the ignition or when you turn on the emergency flasher.

But when you apply the brakes, only one side lights up. I think it has to do with the left turn signal. The right brake light will flash when you signal right but the left brake like stays solid when you signal left. Otherwise, the left brake light doesn't light up at all when you apply the brakes.

Any ideas what could be wrong? Easily fixed problem? I'm no automotive genius but can do minor fixes.

[4X4]

Alot of those older bulbs have two filaments in them. One could be out, which would screw the proper function of whole bulb since they run on series circuit.

[CrusaderPi]

Did he check the fuse?

[Hack&Lube]

Fuses have been checked. Interesting note about the dual filaments. Will maybe swap bulbs from side to side to make sure.

[Mrs. Rathji]

Could be your front signal that is burnt out causing the malfunction on the rear. Another thing to check is if the signal light in the car when driving flashes or stays solid. Flasher could be out if it stays solid in the car.