he first thing to check is the 15amp fuse located in the cabin fuse panel. If that is ok then you will more than likely have to change the brake light switch. It is located at the very top of the brake pedal assembly.
If it ends up being the bulbs, see sources and related links below for bulb info.
There are two screws under the back dash that need to be removed to lift the assembly off the back dash. I had a problem with the same car. No brake lights at all. I disconnected the power to the third light and the main two brake lights started to work. There must be a short in the third brake light harness.
it would be the brake light switch about 20 dollars at autozone
The 2008 Kia Optima has 16 valves.
4 screaws remove lences
The 2008 Kia Optima is a gas-powered vehicle.
The 2008 Kia Optima has a 5-speed manual.
The 2008 Kia Optima has an inline 4 engine.
The curb weight of the 2008 Kia Optima is 3142 lbs..
The 2008 Kia Optima is 15 ft. 6.4 in. (186.4 in.) long.
The 2008 Kia Optima has double overhead cam (DOHC).
The 2008 Kia Optima's turning circle is 35.5 ft..
The 2008 Kia Optima runs on regular unleaded.