What you're looking at is surface rust on the bottom of the left side rear frame rail. Jack the car up, and drill three more-or-less evenly spaced 3/8" holes along the bottom of the rails on both sides from the engine mounts to the rear crossmember. The spacing isn't critical but there are doublers in certain areas that you'd do well to avoid, as its just more metal to drill through.
What happens is, the big oval holes in the rail tops catch rainwater thrown up by the rear tires, and the rails have no way to drain it off. So it sits in there and rusts as it evaporates. I've seen rails absolutely full of rusty water. After drilling the new drain holes, some guys push corks in the freshly drilled holes then fill the rails with drain oil. They let it set for a few hours, then pull the corks & let it drip dry. That coats the inside of the rails to prevent further rusting.
Next, with the rear wheels off, drill a single 3/8" hole in each lower leg of the horse-shoe shaped stampings that supports the rear a-arms (4 holes). Try to get the hole as close as possible to the bottom of the stamping near where they are welded to the rails but don't punch through the rails themselves. You will likely get a cupful of talcum powder fine dust, rust and who know what out of the horse shoe stampings. This mix turns to mud in wet driving.
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