I just bought this 2005 Ford Ranger FX4. I was replacing the heater core and I noticed this (see pictures below) when I was removing the carpet. I need so advice on what to do about it. I can stick my screw driver threw most of the driver side floor.

 

Wow thats pretty bad, looks like 3rd brake light was leaking for a long while, or maybe a door gasket

Looks like a 4 door cab
Not sure if anyone makes replacement floor pans for these anymore
Might need to find a cab in wrecking yard and cut out the section you need and weld into yours

 

Mine is like that but way worse. I have the same area affected in the back and pretty much the whole front floorpan.
I'm in the process of cutting out all the bad metal to weld in some new metal i bought. I also ordered a 92 floor pan for the front and the fit is pretty spot on and mine is an 04.
I'd say hit all the rusty area's with a metal brush wheel so you can see where the bad and good metal are, make your lines and cut the cancer out.
I left those small pieces of metal where the rear seat bolts were so i don't have to deal with lining those up right later on. You can see what I mean in the pictures.
Make sure you rip off all that rubber coating covering the floor. Mine had a lot of bad spots under it. Also check the passenger side while you have the carpet out. Mine was rusty but nothing had rotted yet so a good time to clean and coat it with a rust converter or reformer and paint. Also, wear a respirator and some eye protection, stuffs nasty.

 

So the earlier floor pans will swap? I can’t find anything for my 03

 

They line up ok pretty decently. Have to shape it a little and do some cutting but you can get it to fit. I ended up using sheet metal as it was easier to manipulate into position and weld in.