The most common type of device fitted to cars when they are wired for towing used to be flasher monitors. The function of a flasher monitor is to provide a warning, visible or audible, that the indicator lights of a towed trailer are working or not working. This is known as a C2 warning.
Some flasher monitors are built into switching relays (e.g. Smart Bypass relays and SmartCAN CANbus modules) and some simply monitor the working of the trailer indicators. Most flasher monitors fit in the boot.