If you look on the movie, Drew is being chased by the killer. She is already stripped of her clothes and runs into the street and sees the car. After she screams "Daddy! Daddy, NO!" she gets hit. If you look closer with slow-motion just before she is hit, she switches from her to a bag in a bikini. You can also see this on the trailer.
Yeah, sure - the director of scary movie (was it Keenan Ivory?), is going to have Carmen Electra be hit by a car going full speed on the road...obviously they're going to use props! Who cares if the bag in a bikini doesn't look like Carmen in a bra and underwear, it was still hilarious!!!
Here are a few comments about your "slip-up":
1:The film makers weren't allowed to actually stab and run-over Carmen Electra, so they couldn't actually use her
2:They would have to use some sort of dummy to show the accident
3:Even though it is very obvious that it is a dummy, it was probably intentionally obvious, as it is a spoof of all horror films after all!
Sure, they are really going to use the real actress to carry on the stunt... it was a low budget, hilarious movie- don't knock it... it was obvious that the person knocked down and thrown in the air was dummy, but any way, thats part of the hilarity! (is that a word)