For the heater you need a 1 ohm 50 watt resistor. Some of the vendors sell them. Here is what one looks like
resistor for blower motor For the fuel gauge you need a 30 ohm resistor of around a 2 watt rating, in series with the supply, not connected as a shunt. The horn will be very loud on 12 volts but you need a 12 volt horn relay.
