All three heaters (Kärcher-Öfen BL 30/U) on the He 219 run on aviation gasoline. The exhaust for the one in the rear fuselage is situated on the left hand side of the rear fuselage, just below and slightly in front of the left stab.
The exhausts for the two wing mounted heaters are located on top of the wings, just outboard of the engine nacelles.
Albeit a model, but you'll see the heater exhausts pretty well on my 1/72 HE 219A-2:
http://modelingmadness.com/review/axis/luft/cai219.htm