They drive the flywheel so they're at the thick end of the engine. They tend to be heavy so they often mount them low down. They take lots of current so a big fat cable runs to them directly from the battery.
Use Google - select images - enter Zafira Starter Motor see the pictures.
Difficult to get to according to this link...
http://www.astraownersclub.com/vb/showthread.php?t=186729
Peccavi, June 2011
