It's not corrosion. It's the residue left over from the manufacturing process. Not uncommon for it to be on one end when the board panels are set on end after they're made...

 

You've piqued my interest ... next time I have a door panel off, I'll examine the switch board for any such residue on only half the board.

Is there an explanation why there's a dearth of complaints about the door switch boards? I'm not aware of any other post where someone has 2 failed door switches. Liquid spills, as the OP thought, are more likely to occur on the front or rear passenger side door switch panels (vs. left rear) - the 2 that failed in his case.

 

The only liquids that I have open in my car is either coffee or soda. Over the years I've manged to spill both on the seats or the floor but never on the door handle although I admit that might be a possibility. Nevertheless, I can't see either of those two substances ruining the traces on a circuit board even if someone managed to soak both door handles.

The thread is interesting though and I will be awaiting what the OP posts further...

 

Over the weekend I needed to open up my front passenger door to remove a piece of broken plastic in the pull-out pocket. While I was in there I decided to take out the switch and see what mine looked like. Here is a picture of the circuit board:



The board is conformal coated with the exception of the circular area around each switch contact. I noticed that on some of the pictures I took where the flash went off this coating reflected the light back and made it look similar to the lower portion of Redmister2's picture. There does seem to be some contamination around some of the switch contacts in Redmister2 board and I can't explain why there would be a break in his circuit board right where his soldering wand is pointed. That being said, the way that the switch is designed, the circuit board is very well protected from spills. Not only does the switch housing surround the circuit board, the switches button membrane covers this side of the circuit board which also faces down. I suppose its still possible that one could spill something onto the switch and you might get some fluids to wick up into the circuit board through capillary action.

To make things more interesting, after I put my door back together again I figured that I'd try my window switch. I don't recall if I ever tried to use it since I first picked up the car back in October. Wouldn't you know it, the damn thing didn't work?!? So I pulled it apart and this time I took the membrane cover out of the plastic enclosure and this is what I found:



WTF??? I was missing three of the four small plastic push pins that force the membrane contacts onto the circuit board traces when you actuate the switch. I looked around my bench area just to be sure that I didn't somehow lose them the first time I opened the switch to take pictures of the board but I didn't see anything on the bench or on the floor and I was pretty positive that I didn't take the membrane cover off of the switch the first time I opened it up. I would have noticed the pins if they had come out then anyway.

So yesterday I took a trip to the local U-pick junkyard and found a switch in the back of a 2004 ES300. Even though the switch assembly as a whole isn't the same (the circuit board on this one is actually much more complicated) the mechanical design is the same. So I took that switch apart and removed the four push pins out of it. For good measure, I removed the pins from another switch and now have some extras in case anybody else needs them.

Put the replacement pins back into the switch and now everything is working like it should...

 

I don't see any of the "residue from the manufacturing process" on your switch board like that which is readily seen on the pic posted by Redmister2.

 

Novice repair work but it did the job.
I got a chance to play with the solder gun tonight. I gotta say I need more experience working with solder. Upon testing I got both switches to light up now, and one is actually working now! I think I'm done messing around and will buy just one replacement and call it good.

 

Great job, McGyver!

 

Great diagnosis tracking down the electrical short, and repair. A used switch board should be widely available and inexpensive.

 

AlRose here. eBay has a multitude of LS4340 2003 parts. I just bought a motor and regulator for £21 delivered.