Does anyone know where to get the cable to open the third door?

 

Junk yard will be your best bet. I picked up all mine last weekend from there

 

It is still available from Ford for about 65 for the latch assembly with cable, I couldn't find any other source for this part. It may be one of those captive parts that dealers only sell. Of course you can get it from the wrecking yard,

I did find a few places that sell a repair kit for the cable. It was 20 from all of them. Good luck.