Yeah, the seat heaters will probably work. The 740 grids seem more durable than the 240 heaters. They also give you a bottom and back arrangement which will help shake the chill... but you'll still need gloves and you'll be able to write your name on the inside of the windshield.
A heater core job is something I wouldn't wish on anyone. Dependign on where exactly the crack is, it may be repairable. Once you have it out, a radiator shop can easily tell you whether they can braze it up or not.
Then you get to put it back in....
Good luck, hope you don't get too cold!
BTW the seat heaters tend to just get unplugged by feet under the seats- check the red/black 2-wire plug under there. Lights are probably just burnt out.
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Rob Bareiss, New London, CT 86 244, 87 244, 88 744