Wiper motors in cars are usually isolated from body with rubber bushing/mounts to prevent transfer of noise to car body. The problem with this is some kind of ground is necessary to make motor work. Usually a wire strap of some kind. This can become non/partially conductive for various reasons and cause intermittent wiper operation.
The rely should be under the passenger side air filter. Remove the filter and unscrew the relay cabin.
December, 2, 2010 AT 1:35 PM
The general module has control of the wipers. A scan of the car with the proprietary BMW scan tool called GT-1 is recommended to uncover the root cause.