I had a diesel air heater on my 1995 SM before I sold it recently.  It is installed in the cockpit main port locker tucked up under the top of the locker.  It is difficult to access once installed since it requires complete removal of everything in the locker.  The diesel exhaust is through the hull.  I didn't use it much given that I was always in warm climates.  But I did use it in South Africa.  It has heating tubes running to the main cabin from under the galley area and exiting under the companionway stairs and to the aft cabin via the engine room and aft head.  Since I rarely used it, the two tiny diesel fuel filters would clog up when not in use and needed to be cleaned before first use.  One filter is on the diesel heater and one is on the fuel line in the engine room where it connects to the main diesel fuel line.