My fuse for tail light keeps blowing

It's called a dead short you have an electrical problem. Take it to a mechanic... sorry a trusted mechanic .

 

The insulation on one or more of your wires has most likely been compromised allowing current to short to ground (car body). Also known as a dead short.

Common areas are where the wires go through grommets at moving doors, etc. Maybe where the harness goes to your rear hatch?

Unfortunately, these problems can sometimes be hard to troubleshoot.

 

Any history with the car being in an accident? Does it happen when going over a bump? How about rodent damage? They can chew wires in the strangest of places.

The dealers will suggest "replace wiring harness" in this instance, which means they don't look for the problem. Very costly. Finding such a short requires pulling things apart and sticking your nose into the nooks and crannies to check for damage.

I've seen a fuse replacement that plugs in to a fuse socket, but has wires attached with a circuit breaker which can be reset. It looked like a very handy item for troubleshooting.

 

You shouldn't power accessories like foglights, etc off of the parking lights circuit.

The way to do that is to use relays that take power from the battery, or in the case of the Prius, from the fuse box in the engine compartment. You can tap into the parking lights for the relay's coil terminals instead.

Edit: If you tap into the parking light circuit to control the relay, make sure you test the wires for current (if they're hot) and polarity both when ignition is off and when in Ready mode. Reason is Toyota often uses a floating ground system where the positive lead is always hot, and the circuit is switched on the ground side.

SCH-I535