Many of the cheaper ones don't give you a fixed voltage anyway. They often contain nothing more than a chain of diodes (for instance 1N400x) and the switch just allows more or less of the diodes to be inserted in the positive line. Each diode drops apprx 0.7 - 1V, depending on current draw. Not exactly stabilized voltage, but it tends to work since most things meant to be powered by batteries have a fair bit of tolerance to variations in input voltage so that they can handle both fresh batteries and almost fully discharged ones.

In the 12V setting, assuming there is one, the diodes might be bypassed entirely - effectively giving you a cig. plug connector, a piece of wire and a barrel connector.

/Michael