Regarding the CAN system it aims to simplify / reduce wiring connections inside the vehicles.
The CAN can be considered as a serial communication network between all units within the vehicle.
For example: instead of wiring the speedo, RPM, coolant temperature, warning lights, etc. signals
independently into the instrument cluster, the whole information can be transmitted on two wires.
The microcontroller inside the instrument cluster will process the the serial data, and sets the
warning lights, speedo, coolant, RPM instruments, etc. accordingly.
Another example: all switches of the passenger compartment (central locking, power windows,
horn, wiper, demister, turn inidcator...) can "sit" on the same two wired bus.
Every switch has a unique ID. The "body computer" on the bus will listen to these IDs,
and enables the actuators, DC motors, turn indicators accordingly.
Some switch IDs may be monitored more than one controller units, e.g.: the signal of the crash detector
is monitored by the body computer (to fire the airbags) and the engine management as well (to disable the fuel pump).
The A/C switch is also one of these.
While the faults of the "old fashioned" wiring can be located with a multimeter (even at home),
the diagnostics of the CAN bus requires special tool to analyze the messages sent through
It didn't fit in a nutshell, but I hope it helps.