With the engine hot, measure the resistance across the plug. It's probably a 400W unit. Divide the resistance into 120 to find the current, then multiply by 120 to find the wattage. Equivalently, divide V^2 by the resistence (120*120/R).

Note that inverters have two limits, VA and watts. VA is volts * amps, and always VA >= watts. The VA rating is usually what is listed in big letters, the watt rating must be found in the specification. VA differs from watts when the load in an AC circuit is inductive or capacitive. Your heater is neither, so watts = VA, and you must buy a heater that supplies both 400VA *AND* 400W. That will probably be a 600VA unit, or higher.