I just picked up a used thule 704 ski rack which
unfortunately came with no installation instructions.
Does anyone know where I can find a copy of the
instructions for this model? I tried thule's web site and nada.

If you ask really nicely at (to pick a 'random' example) one of the coop chain shops, they will make a photocopy for you from their floor stock.

Did I mention 'really, really, nicely?'

The Thule site has some info but not on the 704, but the 725 and 726.... You may want to check these out and see if they apply to your predicament at http://www.thuleracks.com/thule/ez_install.asp