Bolts through the roof will probably be your best option in the rear. On each mounting foot are those two rubber pads. Each rubber pad is held on with a nut, a washer, and a carriage bolt. If you can get them apart without breaking the bolts in half, you're a lucky man.
Take the old hardware out, and get some new hardware. Make sure everything is made of stainless steel. Get some longer carriage bolts, some flat washers, some lock nuts with the nylon inserts, and some fender washers. It may be obvious, but the fender washers go inside the car to keep rest of the the hardware from pulling through the roof.