How are you this morning, fatman,
Here's some ways that I've done it, but certainly not the only ways... experiment with the ideas you'll pick up here and see which one works best for you.
You can simulate rusted out panels by grinding away plastic on the inside of the body with a Dremel grinding wheel attachment. You just make light back & forth motions until it starts breaking through, leaving rough edges around the hole, then weather it to a rust finish when you're working up the setting.
You can heat the plastic, then compress it with something, to simulated a wrinkled body panel, but practice first because too much/too little heating will cause it to look like melted plastic instead of bent metal. An alternative, is cut away body parts, and replace them with heavy duty aluminum foil, which offers all kinds of possible damage options.
FSM has had several articles pertaining to this over the years. It's an interesting technique that produces some very realistic results.
Good luck,
Take care,
Frank