The son has a 89 that has a rusted out floor from that on the driver's side. A couple of places to check are: There's a round metal plug in the floor at the driver's feet. Make sure it's sealed good. Then under the hood below the master cylinder where the fender well slops down in the rear and meets the firewall there's a joint, make sure it's sealed well, his had hardened and cracked. Then underneath in the same area of the firewall there's a seam in the floor pan that was not properly sealed. As another reply said it could be a windshield seal, expecially if it's been replaced and was not done correctly, it's glued in. Also under the master cylinder where the wiring harness plug comes through the firewall. It's a plug type thing with a small bolt in the center of the outside part holding it on to the fuse panel. Also there's a couple or things, like wipers, under the grill at the base of the windshield that could be leaking. I use a fiberized roofing patch, about $5 for a gallon at HD or Lowes, because it stays flexable.
JoBo