It sounds as if your brake light switch is bad, or, your brake pedal return spring isn't pulling the pedal back towards the seat.
First, just pull up on the brake pedal, opposite direction of when you are applying them & see if the lights go off. If yes, you'll need a new return spring. If not, then usually the switch mounted up above the pedal is defective & needs to be replaced.
I believe once that is corrected, the lights should work properly if your parking lights are working, as they both use the same circuit. If not, you'll have to do some testing of those wires & backtrack to the light switch.
The Brake light switch also can have the cruise control switch & other controls associated with it but not always, depending on the specific Model & Year & other options so testing it could require a wiring diagram, I've included a link that will give you the wiring colors for the brake light switch itself. Testing the wiring is the only way to determine any other condition that would be causing that to happen.
Here's the link for the wiring:
http://www.commandocaralarms.com/wiring/2003-2005/Kia/Optima/893.html
It shows the brake switch wires as GREEN.
Good Luck.
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