This is the brand of car dehumidifier that I have and as you can see they also sell for about $20 a silica gel dehumidifier and to me, if the inside of your car is dry then it doesn't matter if you have heat coming out because there shouldn't be enough moisture in the air to fog up the windows.
Calculations to figure out of course would be to see if a 1000w block heater does more good then a 1000w space heater (Ben was using an oil filled space heater in his electric metro), of course the block heater is going to allow you to get better mileage and with luck have heat right away, where the space heater is just going to help with heat and maybe defrosting windows.
From my figures last winter, plugging in a block heater used less energy then running a cold engine, so I think you should install that block heater while the weather is nice! I would do the same thing in my civic but I broke the last to sockets I had when trying to remove the plug where the heater screws in to.