The most common type of device fitted to cars when they are wired for towing are 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.
The most common are the simple boot-fitted audible monitor and the Smart Bypass relay described in the section on Bypass Relays. Front fitting flasher monitors are less common than they used to be but connections are often made at the front for power supply and, in the case of SmartCAN modules in some vehicles, connection to the vehicle's CAN (Controller Area Network).
See Flasher Monitors for full range of relays and fitting details.