A thermoelectric generator utilizing a number of thermoelectric modules to produce electric power from waste heat when waste heat is available and from an alternate heat source when waste heat is not available. In both cases hot gasses are directed along several separate paths so that all of the modules are provided with approximately equal hot side temperatures. In a preferred embodiment the engine exhaust exits into an octagonally-shaped plenum into eight separate heat exchangers. Eight modules are mounted in each of the heat exchangers for a total of 64 modules. Preferred embodiments of the present invention include an auxiliary combustion burner-blower unit producing a hot exhaust that exits into the plenum to provide electric power when the truck is not operating. Valve features are provided to control the temperature and exhaust flow rate through the generator.