Why won't my Car Start in Cold Weather?

My wife's Santa Fe runs fine but whenever the car sits out overnight in below
freezing temperatures (we don't have a garage) it won't start.  To start it I put
the car in park, turn the key to run, pull the starter relay and use a piece of
wire to jump the starter.  It starts every time. I've had it towed to the dealer
multiple times and they can't figure out what causes the issue.  A tow truck
driver showed me the starter relay trick.  I've replaced the starter relay 3
times, the starter once, and the battery twice (even though it was good) but
the problem is still the same.  Once the car is up to temperature, it starts
normally.  This happens every winter.  Any ideas?