I thought I'd do a 2-hr pool noodle job on one of my driver's seats. Pulled the seat and started to vac the L rear footwell when I noticed some minor rust (like they describe in the eBay ads). This was around the LR driver seat support. Didn't look bad. But when I probed with a small flat tip, the blade went through. Kept poking and probing 360 until I found solid metal, etc.

That was about 8 hours ago. As the pix below show, getting a MIG welder is the best investment a W123-er can make. (It's obvious I'm still learning how-to.)

Couple of points:
1. Both rear foot wells catch water as the low points in the car. Pull the carpets and check these areas.
2. The foot wells usually fill from "uphill" leaks in the front windshield or clogged engine compartment hinge drains. Keep checking those.
3. OEM/subsequent undercoating may protect the underside of the car from wet corrosion but it also does a great job of keeping any leaks inside the car, futher accelerating metal oxidation (rust).

Good luck withj yours.

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