The tail lights stopped working on my 2005 Prius. The manuals and dealer service department were helpful. Some advice would also have been helpful. Two things need checking. The fuse box and the light bulb. Fuses are located in a fuse box that is under the dash on the driver's side. You have to lie on the floor with a flashlight to find it. You have to remove a cover to reveal the fuses. The manual shows which fuse is involved for the tail lights. It is a 10 amp fuse located at the end of the assembly. A fuse puller is required to remove the fuse. A puller is mounted in the main fuse box in the engine compartment. The fuse box has a removable cover. The fuse puller is white and inserted into a special location described in the manual. The fuse can be checked with a continuity checker. I have this available on my inexpensive ohm meter (Volts, Ohms, Milliamps or VOM meter). In my case the fuse was OK. Next check out the bulbs. A cover can be removed from inside the back of the car in the corner where the rear corner lights are located. The tail lights are the lower of the three bulbs. The socket can be removed by a a quarter turn counterclockwise. Without stressing the wires, slide the socket out enough to remove the bulb and after checking the bulb with the Ohm (VOM) meter replace with a new bulb as required. I found the bulb, #194, at my local auto parts store for $4.95 for two bulbs. The socket is replaced with a clockwise quarter turn and the cover is replaced by noting the way it is attached to the lining in the rear of the car.