Should be hot with ignition on.
The motor "+" lead is shown as a black/green wire on the schematic I'm using. It comes from fuse #5. The black/green wire supplies power to the motor from the wiper switch.
+12 volts is fed to the wiper switch by another purple/black wire that also comes from fuse #5. See if you have voltage on this wire at the switch.
Here's a link to the schematic.
Print out a copy - you can mark the leads you're trying to trace -
http://www.bmw2002faq.com/component/option,com_forum/Itemid,50/page,viewtopic/t,316840/