I just don't see any 12 volt heater as being very practical. The OEM heating element is over 4Kw and even a 1Kw 12 volt heating element would draw about 100 amps from a 12 volt source . . . . and using only 1Kw isn't going to get you a meaningful amount of heat, let alone defrost - It might take a long time to get the windows clear. . . . . . and how long can your used 18650's put up with a 100 amp draw??
You say you use the car for short distances - What's wrong with using the stock heating system just as it is??
2012 iMiEV SE Premium, White
2012 iMiEV SE, White
2017 Chevy Volt Premier
2014 Ford Transit Connect XLT SWB wagon, 14,000 miles
1979 Honda CBX six into six