There's a HUGE difference between "not working" and "missing". See where I'm coming from?
Two ways to fix it. The ugly way, and the right way. The ugly way is to just buy a universal one from the auto parts store. Depending on what happened to the original light, the wires may still be in the general vicinity. You should try to find those, as the way the Taurus is setup with integrated brake/turn signal bulbs, it's harder to do it correctly if the right wires aren't there.
The right way is to visit a local junkyard or eBay and buy a used one to install.
Can you check in the trunk and see if the wires for the 3rd light are still there?
Here's one for a '93. I would guess that any from 92-95 will work. Getting the right color might be harder though.
Incredibly long URL
A nudder 1
There's 3 more at least. Good luck.