Where does that bolt come out on the other side anyway? Into a threaded plate somewhere under the trunk panel?

It appears to be captive in a box that is welded to the frame. There is a rubber plug on the inboard side just above where the bolt would be, but if you stick your finger in there, it is just sheet metal on top of the bolt hole. I sprayed the hell out of it wit PB Blaster anyway, but I don't think I even touched the top of the bolt or the fitting.
Now I am off to the parts store to find a tap that will fit. I am pretty sure it is M12 but don't know the thread pitch. Hopefully they can help me with that.:confused:

Check for the pitch on the bolt - I'd expect it to be 1.5mm though don't quote me on it

Here are a series of pictures of some rust repair on this same point on a S124. They might be a helpful reference for what you're working. Another possibility is to simply cut out the offending piece and weld in a new one.

http://jan-wulf.de/wp-content/uploads/2015/01/tee-1.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-2.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-3.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-4.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-5.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-6.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-7.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-8.jpg
http://jan-wulf.de/wp-content/uploads/2015/01/tee-9.jpg

Credit to Blecharbeiten | jan wulf

No where did I see you threatening the township for repair money. Public road, their responsibility! Sue the bastards!!
Anziani
'97 CL600 51K