If you keep your compressor working with no aux. fan, get ready to buy a new compressor. Do a couple of things:
1. Run a line from the battery to the aux. fan, be carefull. If it works, it is not the fan itself.
2. Check the freon pressure on ac system. If it is low on freon, you most likely, have a leak somewhere and that is causing the aux. fan not to work.
Did this happen all of the sudden?or was it a slow leak?
If not familiar with ac system, take it to a good MB mechanic as soon as possible.