Like the falling leaves, the migration of songbirds, and the end of daylight savings time-- this topic comes around every year.
Whether or not an electric heater will be enough for you is really a function of what you expect of it. I have an Espar gasoline heater, similar to Danny's (about 8000 btu/hr). That's a lot of heat (enough that the heater often shuts down here in the snow-belt because it gets too hot). For the sake of discussion-- if you have the expectation of driving with the top up, down to 30*
OA temp (or so), and you don't expect this to be 80* inside, you could probably get by with 1/4 of that.
With electric, 1000 w equals 3412 btu/hr. Ohm's law says that with a resistive load, volt x amps= watts. Your alternator makes about 14 v (+/-), so 1000 watts would be 71 amps. You don't have 71 extra amps to work with, and nobody wants to run welding cable to their heater. 1/4 of 8000 btu/hr (what the gas heaters throw) is 2000 btu/hr, or 586 watts. The heater that you want will be at least 500 w, which will require 35a at 14v. Amazon has a few 500w heaters for short money, but the reviews are not good. The guys who took the time to check current draw noted that they were putting out half of what they were rated for.
A decent heater is going to be a few hundred bucks. It's going to need to be hard-wired and is not going to plug into the cigarette lighter. If I lived in Phoenix, I'd get a 12v heated blanket, wear a jacket, and love life.
I can't imagine how cool having a speedster as a daily driver would be.