What part of the transmission box controls the reverse lights?

In an auto trans its called a Transmission Range Selector TRMS. In a manual it's called a park/neutral switch. It's in different places on the transmission depending on which model focus you bought and which engine is in it--which you didn't tell us.

Getting the point? If you want people to help, you have to give good info like year, make, model, and engine.

if your reverse lights are operated by your transmission, itll be a bolt in sensor on the trans. modern standards the reverse lights and neutral safety can be mounted inside the shifter instead. anyway, itll be easy because the manual trans generally only has 1 or 2 connectors on it anyway.

I suppose some car might charge a list price like that. The part could probably be had in a junkyard for $2. It just screws in and wires are connected to it. So it is just the one wrench or tool.

without knowing what kind of car it is some are in the side of the transmission some are actuated by the shifter being placed in reverse and others in the stearing colum like fords

The switch on most box, located on the box itself, and required no skill to replace it, just remove the wire, and unscrew the switch with a proper tool.

HAd you told us what kinda car we were looking at, we could be more help.