The rear bumper sounds like typical bumper sag to me a common problem with our cars. The sheet metal buckling is another. I keep mine in a garage as well but it is insulated ad heated so the nasty cold doesn't get to it.

99 CSVT Green/Tan # 84 of 2760 Built on Wednesday, September 30, 1998 15.376 @ 92.00 MPH Stock 75,000Km 96 Contour GL 2.5L V6 ATX 170,000Km