The goal of this ible is to show you how to wire in a 4 wire trailer plug into your tail lights. Installing a hitch is fairly simple, and even if you have an accessory hitch (for bike racks etc.), the electrical harness is a big up-sell by some installers. Yet, you can do it in under an hour for under 25$.

Full video now included.

Step 1: How It Works

You will need to buy an inexpensive box that takes the 5 wire input, and changes it to the 4 wires the trailer plugs use (essentially it merges the turning and breaking signals on a single wire, while preventing a short on the car's since since it uses two different circuits there).

Step 2: Material

You will need

A 5 wire to 4 wire converter so that you can use the tail light signals for the trailer lights. One that won't require extra cable purchases and all should go for about 20$
a soldering iron. A battery powered one could be enough, but I had a bit of trouble in the end and ended up using my butane one.
a utility knife
snips
heat shrink tubing
a flat head screw driver
an ajustable wrench
a wire stripper (optional)

Step 3: Remove the Lining

There are clips around the lining, which you simply pop off. In my case I had two kinds, you will most likely have one of the two or both. You should focus on the corners, make sure you remove enough to be able to peel away enough of the lining to have enough room to work in.

You will also have to remove along the edge if you want to hide the one wire that crosses over to the right side (more on this in the wiring stage).

Step 4: Figure Out Your Wiring

The exact wiring will depend on your car model. You can do this with documentation, or by testing with a multimeter. I did both.

In my case I googled it and found a list of the wire that correspond to the break, turn (x2), ground, and running lights. To make sure everything added up, I connected the negative lead of my multimeter to the ground (the body of the car), and touched the pins to measure if the voltage changed when I was using the function (break, turn signal).

In case you have a yaris as well, here are the wires.

ground: the screw in the body of the car (as for all cars)
break light: the purple wire (back), from the front it would be the bottom right pin
left turn light: teal wire (back), from the front, bottom left pin
right turn light: on the other connector (right tail light), it is in the same place as the teal wire, but it was yellow
running lights: the green wire (back), from the front, that is the top left pin (not the empty one obviously, the one next to it)

Step 5: Splice the Wires

Once you know your wires, splice them. It goes as follows:

snip the wire, strip both ends and the corresponding wire on the unit
twist the wires to make your life easier
slip tight fitting shrink tube on both of the ends of the wire you snipped, as well as a oversize one (about 1/4 inch) on one of them
twist all three together, and bend one back so it's a line, not a bend
solder
push the shrink tube in place
heat the shrink tube

The exception is the ground that gets put on the grounding screw.