My 1989 Defender 90 lacks a rear wash wipe system, Does anyone market a suitable retrofit kit? Would prefer to keep the system separate from the front wash wipe system to avoid having to remove the roof lining

?? your gonna have to run some cables back there for the switches, so why cant a water pipe go in the same place??
have a look round, get some fender rear wash wipe bits, and robert yer fathers brother, if you really do want the water bottle in the back, theres plenty of "universal" bottles you could use, some even have the pumps on.