On my 11 about 10 years ago I fitted wiper mounted washer jets that came from a Peugeot 405 that was sat in the scarp yard back in a time before full blown health and safety when you could go and rummage around the scrap yard yourself pulling things off the piles of car. I said then that it was the best modification that I had done.

As some of you may be aware I had an engine fire in the 110 and am in the process of rebuilding it (thread). in the mean time I am using my mum's td5. I have just done the same modification to this using the spray bars found here: link as the scrap yard as long since closed. All that is required to do the job job is the kit linked and and a y-type tube fitting bought off ebay (link).

Once you have the parts you need to remove the dash top, which is just a whole lot philips screws to give you access, remove the pipe form the current wiper jet (leave it in place to seal the hole) and use the y-peice to turn one hose into two. You can then feed this through the bulkhead holes used for the left hand drive wiper positions and run it up the wiper arms to the spray bar that is cable tied in place.

Although the pressure is not high as you have split the run it does not need high pressure as it directly onto the screen. I have just bought another kit to fit to the 110 as part of the rebuild and especially at this time of year it remains one of the best and simplest modifications I have done to my defender.

I did similar to my 90 as the spray jet was always blocked or just sprayed water under the wipers barely touching the windscreen. I had a T piece out of the bulkhead and the tube cable tied to the wipers which had some holes in to spray the windscreen.
Did away with the rear door glass and replaced it with aluminium (van body), so I wired up the rear screen pump with the front so it had double the power and actually managed to clear the screen.
Simple and effective mod