Welcome to Dr. Björn's Auto!
This custom aluminium tank fits underneath the spare tire of a Jetta Wagon (early 2000s). It comes with an integrated heater and fuel gauge. The plastic tray above the tank will have to be modified in order to accommodate the spare tire. Once installed tank and spare tire are hidden without any impact on the rear compartment.
Note: The tank in the picture is an older prototype. The new tank has fittings for fuel and heater on the rear and a different filler.