The reason they rust in this fashion is due to leaks. Water often enters on the firewawll and runs down to the drivers side floor. Once the padding and carpet in this area is drencehed it runs to the passenger compartment via a channel that wire are routed through. You need to find where the leaks are and repair them, then fix the rust damage. Try looking through the archives for rust repair. Some of the guys have used a product.... POR 15.

I currently have the carpet and padding pulled out of my 1984 trying to find a leak. I think mine is from the airbox. When it is humid and I run the A/C the condensate leaks out of the seam on the air box and gets the drivers side carpet wet. However, by running a hose on the car I did find 2 other areas where it was leaking. One area was where the hood release cable runs through the fire wall and another area where there was 1/8" hole in the fire wall. I filled both of these with ATV silicone and I think they are sealed.

Another place old Volvos leak is a missing or rotting plug on the firewall. The plug is located on the drivers side up near the firewall in a gusset up against the fender. Take a flashlight and shine it up in this area and see if the plug is damage or missing.

I have looked at a lot of older bricks for sale and they all usually have some evidence of leaks.