There's a mod on this forum to replace the rear wiper switch on hatchbacks for estate switches & relays. Have a look through if you want to do a proper job. I'm intending on doing it on my car when I can get hold of an estate switch.
Alternatively, you could do a quick 'bodge' for the time being by just linking 2 of the grey wires together with a toggle switch to make the wiper stay on. (It's the top left & bottom right grey wires in your picture). You could even secure it where the fog light switch would go, but if you did it this way, it'd just be on or off, there wouldn't be a time delay.
The Peugeot 205 uses the same sized switches as the BX for certain things, such as the fog lights, so they'll push straight in, but they look different from the front.