"just got a new wiper motor from paddocks for my 1982 Landy 90"
You must have your vehicle age incorrect, the 90 wasn't made in 1982.
RTC3867 is certainly correct for the earlier vehicles (not certain if / when it changed) as I fitted a new one to my own 1986 90 earlier this year.
It does sound as if there is a problem with your switch, probably worth checking with a meter or just fitting a new switch while you are at it (cheap enough). Without knowing the correct year of manufacture, does yours have int wipe as a faulty relay or poor connection can cause strange symptoms?
My new motor was an OE unit (Bearmach boxed but manufactured by Lucas) and when I fitted it I discovered it was operating in reverse (unfortunately I had already fitted the wiper-arms so had to kill the power ASAP).
A quick Google search soon brought up that it is not uncommon for LHD spec motors to be supplied in the UK but luckily it was the work of just a few minutes to move the ramp (which operates the park switch) on the drive-wheel to the opposite position which resolves the problem.