As in maybe something happened at the party.. could she have been knocked out there and driven to this location? If so.. did whoever driving the vehicle back into something at the party location?
I'm not sure about the party but this scenario makes the most sense to me. If she encountered someone either planned or random where the person gets in her car, this person would likely not just so happen to have accelerant on hand. but if someone needed to move her in order to distance themselves from what might have happened somewhere else then I can see accelerant being acquired before then driving her away from initial crime scene in a very sloppy attempt IMO to destroy evidence. The reclined passengers seat could be where she layed while in transport.