Electric car heaters don't burn fuel, so users don't need to crack the windows when using them. Heaters that use propane and other fuels, however, require some ventilation to use safely.
Electrical heaters run electricity across a wire designed to heat up. This process doesn't produce any chemicals in large quantities, so they're safe to use in cars. However, space heaters designed for ventless operation that burn fuel often produce relatively small amounts of carbon monoxide and other chemicals. While using them in a room does not generally pose a threat to safety, cars have significantly less interior volume, so the small amount of carbon monoxide can build to toxic levels in a short period of time.