While you are checking the rocker panels you might want to check the back seat floorboard wells. This past weekend while replacing the transmission mount in my wife's 240 I noticed several small holes around the passenger side well. Yep it was rust. This in a car that has never been out side of Alabama (except for the first 5000 miles which were spent in Germany). The undercoating still looked solid, but evidently it is trapping water some how. I checked the rest of the floor pan and found no rust anywhere else.
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1988 740 Turbo 203k, 1986 245 275k, Gone: 1962 122-wish I had that one back.

and don't forget to check the exterior rear wheel wells. My '86 245 had several rust holes in there which allowed water spraying up from the wheels to flood the rear floor section.

figured this out after fixing my rocker panels/rewelding new floor metal in/extensive rust repairs/excessive cursing .... can't go on

good news is I'm now dry on the wettest of rainy days!

jack
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Bad Blue, '86 245, 253,000 mi., Columbus, Ohio

"Where do I have him start looking?"

Reach under the car along the rocker and press along the seam between the floor and the rocker. If it's soft because of rust, you've found the problem.

The grill in front of the widshield directs fresh air and leaves, dirt, water down the side past the fresh air vent and down through the rockers to the rear wheel well. The stuff gets trapped in there and causes rusting along the seam. If the weep holes in the rockers aren't clear the water enters the cabin.
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1980 245 Canadian B21A with SU carb and M46 trans