If the warning light is only on when the park or headlights are on, it's one of those circuits. If the warning light stays on even if you turn off the park and headlights, it's the brake lights at fault.

For park you have front corners, rear corners, tail lights and license plate lights; two bulbs per. It's typical for a rear corner bulb to not illuminate because of a poor design on the bulb holder. For headlights you have the headlights (duh!). For brake lights there are the pair of brake lights in the tail light cluster and the center brake light in the cabin.

Sixto
87 300D