Assuming it was Perry who drove the car to where it was found, if the car door was left open, my sense is he exited quickly and/or under the influence of something since even a distressed person would likely close the door out of sheer habit. Unless he had the wherewithall to stage the appearance of a quick exit.
So. He may have jumped out to flag down a truck thinking it would be a) most likely to stop for a hitchhiker and b) then travel the furthest away; or an aquaintance picked him up. Bearing in mind that if he went straight from the theatre to the 'pickup' spot a rendevous would've first entailed stopping at a payphone in 1974. Plus, it would make more sense to meet a friend where they'd be less likely to be seen/remembered, as in not w/ 2 cars on the side of a highway. In any event, he would've had the time, but had no way of knowing if there was a BOLO on him. Much of that depends on the details of the fender-bender. Incidentially, I think it unlikely he was pulled over by police and met his demise w/ their assistance, although it has happened. It would've had to be an officer willing to risk offing a 17 yr old w/a minor record, who knew who Perry was in the first place.
More likely, a 17 yr old in a panic and/or impaired might go straight home from the accident. If he truly wanted to leave anyway, that would be the time to pack belongings, esp crucial meds, extra glasses, etc. I don't see any mention of items missing from his home. On top of that, assuming he still lived at his parents' home, he'd have to be sure that everyone was out for at least the immediate future unless he regularly came home, grabbed things, and left again. It'd be helpful to know the travel time/direction from the fender-bender site, to where he lived, back to where his car was found.
If he was able to pack, he'd be slightly more prepared to leave his car and ride off with a stranger or friend. I find a friend less likely as the majority of people, especially teens, would crack and tell what happened once the immediate dust settled. Or even call in an anonymous tip.
If he did take off, did he do it in the middle of a Michigan winter's night w/ just a backpack or worse just the clothes on his back? With no exact planned destination (that we know of) did he have the $ for a hostel, etc. or was he planning on sleeping in moving trucks; if he didn't leave that night, did he have the resources to take a breath hidden by friends before he took off (knowing the clock is ticking in the search for him)?
People take off and start new lives. But the totality of this being seemingly unplanned for that particular night, his being 17, the weather, and the laws of probability point to him being deceased. I don't think he was chased down by whoever he hit, if there's a record of the accident he/she would've been questioned in light of Perry then going missing. And what are the chances of a second car, perhaps driven by a friend of the driver who was hit, heroically chasing Perry down? Esp since it seems Perry talked to the driver of the first car giving him/her time to get his tag #. Too complicated, and the answer if usually more simple. My guess would be fled from the car and died of exposure, or he was picked up and met with foul play by either that driver or later on wherever he ended up. Also, I hope I'm wrong.
Lastly, on the cartoon voices...could the brother have just received a random, run-of-the-mill prank call by a kid throwing his voice? The brother's reference would understandably be 'cartoon' hearing a certain pitch/inflection, especially with Perry probably never far from his thoughts.
