1999 Toyota Corolla Rear Windscreen Wiper

All of a sudden last week the rear windscreen wiper has come on all the time in my car. Can't switch it off at any point on control switch. Front windscreen wiper which is oprated from the same control stock is working fine and can be switched off & on ok. Just a problem with the rear. Only way rear windcreen wiper will go off is when ignition switched off. I eventually had to disconnect the lead to the rear screen wiper directly from within the hatch-back so am currently without the use of the rear wiper. Model is a 1999 1300SE 5 door hatchback. Any guidance as to this probelem would be appreciated. I have a new battery and the alternator was checked at the time this was fitted and was tested as ok.