You have a short somewhere.
The purpose of a fuse is to be the weak link in a circuit. If more current is running through a circuit than it was built to handle, a fuse will self destruct and break the circuit, preventing overheating and possible fire. The most likely causes of a circuit having too much current are too many devices drawing too much power on a wire, or a wire that is connected directly from power to ground with no components providing resistance.
Since you say there are no addons in the vehicle, the circuit should be in stock condition. I would assume somewhere on that circuit the 12v power wire is being grounded. It's safe to assume that it's happening after the fuse panel, somewhere before the circuit returns to ground. You're going to need to check the wiring along the circuit and try and find it. Most likely a wire has been damaged. A wiring diagram for your vehicle will help immensely, as you'll be able to see what other components are powered off that fuse, and where and how they branch off. Otherwise, check for anything else that doesn't work. Dome lights, glovebox lights, and trunk lights are commonly on that circuit, and they can all ground in different locations.
The 12v cigarette lighter is a common source for power when installing addons. I know you said your car doesn't have any now, but it's possible it did at some point, and then they were removed. I'd check the back of the cigarette lighter first and ensure it looks ok. You may even want to check the socket itself with the continuity setting on a multimeter to ensure it hasn't shorted somehow.