There are 2 heaters fitted to the cab, one for the driver and one for the passengers. The drivers' heater works just like it does in normal cars - there is a heat exchanger under the bonnet that blows hot air through tubes into a box under the dashboard. From there it is sent along more tubes to the window demisters, or comes straight out of the box onto your feet. In my cab, the tubes had come loose at both ends, so I wasn't getting any demisting at all. 4 cable ties fixed it.
The rear heater works a bit differently. There are two hot water pipes that run from the engine and along under the centre console to a heat exchanger box which is under the driver seat. The box also has two big fans in it which blow hot air straight into the passenger compartment.