I borrowed the below heater to try last weekend in our hybrid to see what would happen. The temps got down to the low 50's/high 40's at night and it worked pretty well. It kept the chill away and us comfortable, but it certainly wouldn't cook us out and much colder weather it might not have been enough.
When I buy one I would I would probably try a different one as there are some that are better reviewed.
http://www.amazon.com/Honeywell-Oscillating-Digital-Control-HZ-7200/dp/B000U666LG/ref=sr_1_21?ie=UTF8&qid=1377565923&sr=8-21&keywords=space+heater