If your garage has electricity, I have had very good luck with "air dryers" for boats.
For example:
http://www.westmarine.com/webapp/wcs/stores/servlet/producte/10001/-1/10001/245832/377%20710/0/air%20dryer/Primary%20Search/mode%20matchallpartial/0/0?N=377%20710&Ne=0&Ntt=air%20dryer&Ntk=Primary%20Search&Ntx=mode%20matchallpartial&Nao=0&Ns=0&keyword=air%20dryer&isLTokenURL=true&storeNum=5002&subdeptNum=12&classNum=380
I use a slightly oversized cotton car cover (the cotton still allows some air movement) and put the boat heater under the car. I have not used one of the heaters that have an internal fan. I didn't want any sparks. The ones I have use about 75 watts, won't burn out, don't have a thermostat--so no sparks and put out just enough heat to create something of a dry envelope around the car. This has worked well with several european sports cars from the 60's whose metal seems able to manufacture rust in a desert.