I don't know what blue box is under the dash next tot he fuse block, but I'm pretty sure you don't need to open it.
The defog relay is on the top side of the fuse block. You should be able to get it it by removing the storage bin left of the steering wheel, and (independently) by pulling out the trim panel on the left of the dash that has the dimmer, mirror control and air vent. This pic was uploaded to show the location of the starter relay (marked by a red arrow). In the pic, the defog relay is just below the starter relay.
The top of the pic is the left side when looking in through the removed dash trim, so actually, the starter relay is on the left side, the defog relay is just right of it.
On mine, the starter relay and defog relay are different colors, so they may be different part numbers, but from the drawing it looks like they have the same socket, so they may be interchangeable
From the wiring diagram, it looks like there is circuitry in the defog switch, it looks like whatever should be turning the defogger off after 15 minutes should be inside the switch. Doesn't mean it's the switch or the relay, at this point it could be either. The relay could be stuck closed ignoring the switch, or the switch could be commanding the relay to always close.
It's relatively easy to remove the trim around the radio, I usually pull the bottom edge out, pivot it up a little then wiggle it to clear the top clips. One test to identify if its the switch or the relay is to pull the radio trim out, and disconnect the wiring connector from the defog switch and turn the key to run position. If the defogger still comes on, then it is the relay, if the defogger doesn't come on then its the switch. One caveat,theoretically it could be some much more unlikely short in the wiring independent of the switch and the relay, but as I said, much more unlikely.