Rusted out rear corner panel wondering if i could spray some expanding foam in the inside of the car to fill the hole. then bondo over the outside?

 

This will not work. It's well documented that the expanding foam that you use to fill in the gaps around doors and such retains moisture. It's make the rust worse. the correct way to do it is to cut and weld that junk out. The cheap way to do it is to cut/sand out all of the crap out, then use something like kitty hair on the inside bridging the gap, then evercoat the outside. (notice i didn't say bondo, because bondo blows.)

Edit, if you want real expanding foam, you can get something called "polyurethane AB foam" it doesn't retain moisture. So you can do that. Only problem is, it gets expensive. TAPplastics.com has it.