Are you saying that both tail lights work, but when you step on the brake pedal, the brighter stop lights don't work? Including the third brake light on your trunk?
If all three lights don't come on, more than likely it is the stop light switch. If the third brake light (on the trunk) works, but not the two bigger ones, you need to replace your bulbs. If it was me, I would try new bulbs first (they only cost about $3) before I try installing the switch.
Let us know. If you need to replace the switch, I'll guide you through.