Doing the winter re-undercoating on my '87, was scrapping of the cracked 
undercoating in the front wheel wells.  The big anchor nut for the seat 
belts was exposed, scraping off more coating showed a rust horseshoe right 
around the damn anchor point, with perforation.  Further checking showed a 
nearby seam where that edge joins to the sidewall above the fenders was well 
rusted/gone for several inches. 
        In a major crash it is doubtful the seat belt would have done a hell 
of a lot of good, tearing out the bolt or the doorsill next to the seat.  
The drivers side is fine.  Rust bias on the passengers side is a sign of a 
town car, driven in slush/puddles next to the snowbanks. Check yours out if 
the undercoat is cracked.
        Peeling up the sill carpet showed lots of road dirt/mud under there. 
This may explain the wet front passenger carpet after when I've been 
splashing in the puddles. Grrr,  it's that PO again, should've resprayed the 
underside once maybe. Check way up in the corners of the front wheel wells 
for cracks/rust, behind the gas filler on 2WD, that's where my seam rust 
started. Look for battery box corrosion also, in the outside corner.  Tim Smith