[vulgar]

I dont know if Id recommend using "great stuff" i think it has a tendency to expand and contract with changing temperatures...to what degree I do not know. If you insist on using spray foam i recommend the kind that the company "DAP" makes as this stuff is easy to remove and doesnt turn as hard as a rock like the great stuff will...so if you do overspray or get it where you dont want it its easy to remove. As far as that plastic piece above the license plate that says alero on it I recommend just taking some packaging foam and putting it behind it. When the subs hit in your trunk this piece flexes out and then vibrates against the piece behind it, making it flex out slightly by using something such as foam, spray foam etc. will cure a lot of that problem. Also putting dynomat behind your license plate can keep that from rattling if it does at all.