The short answer is that you can NOT heat your vehicle using an electric heater. The long answer is as follows: Theoretically you could power it with a 12volt DC to 120 Volt AC power converter capable of producing at least 1000 watts at 120 volts. I say "theoretically" because in reality, that little 900 watt heater wouldn't do very much to heat a moving vehicle, and your alternator could not handle anything much larger. Also the power convertor would not last very long running an electric heating element -- if you read the fine print for these converters they usually specifically say they should not be used for anything that contains a heating element. Heating a car with an electric heater is just not practicable. If you really don't want to spend the money to fix the standard heater, the only real alternative is to install an auxiliary space heater that uses the vehicle's hot coolant just like the oem system. They sell these at JC Whitney and they are not too difficult to install for someone with medium-level mechanical skills and the tools need to cut through sheet metal, route and secure heater hoses. It takes some creativity and fabrication skills, and a willingness to do a little butchering on your vehicle -- which is usually not a problem if it is near the end of its useful life. Here is a link. If it does not work, go to JCWhitney.com and search for 12,500 BTU AUXILIARY SPACE HEATER. It costs $145, and you also need the installation kit and hoses.
http://www.jcwhitney.com/autoparts/I...D:100000219815