There are very knowledgeable toyota mechanics on this site who can & will answer your question, but your gonna have to provide some specifics. These guys are good but it would be hard for anyone to answer such a vague question.
Is the light behind the dash? Mounted in the dash face? Is there power to the wiring & if so when does the light operate?
Most importantly, what year is the truck?
My (uneducated) guess would be that it is either an ashtray light or backlighting for the heater control panel.Not positive, but I think there may also be a light inside the cigarette lighter itself (so you can see to shove the cig lighter back in there at night without melting your dash).
In this case it might be helpful to post a couple pics.
You can also search the forum for "FSM" and you will eventually find a link to your truck's factory service manual. Invaluable when you're up to your elbows in wiring.