Shaft from engine to under seat - friction on plate under seat. Friction near engine, transferred to water or air, then ducted to passengers. Heat pipe off of exhaust system, near front seat. Blow hot air over the exhaust manifolds - to passengers. Flame under seat that heats plate - contacts seat cloth.
Flame heats hollow tube through which air or water flows to passengers.
Flame radiates directly onto passengers. Hot wires in the seat. Hot wires in the heater ducts. Have a focused problem to solve. Small town atm.