Product review (4 stars): Condenser Fan Replacement for SubZero 680/S
We used this kit to replace the condenser motor fan on a SubZero 680/S;  the fridge is a 1999 model and we noticed the wall between the fridge and freezer getting (very) hot, and the freezer not holding a temperature, so a quick look in the top of the unit showed the fan not running at all!

The thing is, if you lightly tapped the fan, it would start again, but then a few days later it would stall out, and you'd have to tap it again, etc - so we replaced the fan motor with this one.

All of the other reviews are right-on;  it's a little bit of work to do this, but we were quoted $500 by our (trusted) repair guy for the part + installation, so it was worth it to do it ourselves.