It seems that tightening the nut is creating some tension in the rack which is interfering with the self park. This may not be easy to resolve, but I would start by removing the wiper arms. Then connecting the nut properly. See if it works properly with no arms to move. The arm position could in theory interfere with the self park.
If it doesn't work with no arms, then you need to look inside the motor to work out what's happening.
The alternative is to fit a shallow locknut between the motor and the rack, but this would only address the symptoms, not the cause.
http://www.theminiforum.co.uk/forums/topic/263094-wiper-motor-testing-fault-finding/Page 42 here on GoMog:
http://www.gomog.com/allmorgan/LucasFault.pdfhttp://club.triumph.org.uk/cgi-bin/forum10/Blah.pl?v-print/m-1237670018/
