a negative potential while its positive input is connected to a power switch 38 the power switch 38 is designed as a normally open relay with an operating contact 40 this operating contact being connected by a line 44 to the heater 36 and by a line 46 to the filament battery 28 the control winding 56 of the power switch 38 is connected on the positive side by a line 48 to the line 34 of the vehicle electrical system and on the negative side by a line 58 to an electronic control unit 60 in addition there is provided between the two batteries 28 30 an isolating switch 54 which being designed as a break contact with an operating contact 64 can isolate the two batteries 28 30 electrically from each other the operating contact 64 interrupting lines 42 66 provided on the positive side between the two batteries the control winding 52 of the isolating switch 54 is permanently connected on one side to the negative potential and on the positive side is connected by a line 50 to the electronic control unit 60 the electronic control unit 60 which controls the control windings 52 56 of the power switch 38 and the isolating switch 54 has several inputs through which the temperature of the internal combustion engine 10 is received by way of a temperature probe 69 the.