Lots of things happen with mini wipers.
Firstly the motor might just be tired. Might need rebuild (Convertable mini's job) or a new one.
The gearbox might be dry and sticky. Take the cover off and check.
The rack tubes could be dry, worn excessively or loose in the pivots. This requires dismantling, but not a hard job
The pipe could be loose on motor nut or the nut loose in motor. Watch it while wipers are working, check for excessive
free play in joints.
The contacts in self park switch might be worn out, or main switch might be poor. This'll cause low voltage to motor.
The wiper pivots might be partially seized.
That should keep you busy.
