Hi,

I'm trying to find the location of the rear window defroster relay. I've looked in the fusebox es in engine compartment and the passenger compartment, but can't find it. According to this it is R5 on the "reverse side"

 

Yes, per the link you shared, it's on the back side of the fuse panel in the footwell. You need to do some work disassembling the interior and unscrewing the fuse box to get to it.
But, if I'm not mistaken, the relay is soldered in place and not easily removable.