Beware where you aim your heater.
This is over two decades ago, but should still apply unless material changed.
Deep in Chicago winter, I had a coworker who used a 1.5kw/1.6kw (100/110V) heater (with fan), on extension cord to warm up and de-ice. The heater was on the back seat. However, while the center was aimed and hit the front wind shield, part of the heated air was blowing on the back of the front seats. The vinyl didn't like it... It deformed. If I recalled, it was probably for over 1hr (time enough to eat dinner) but under 2hrs.
Car's interior heater is made for the car. If I have to use an external heater, I'd go with much less powerful heater if there is a possibility that you leave it running long.
EDIT: adding this:
Just in case you wonder how he could plug in the power at the restaurant... We were working there, college jobs back then.