I found this a while back but folks have had mixed success. It may depend on the year of your car and what software is installed.

This little Easter Egg can tell you if you have any DTC's stored. Unfortunately, it does not give the actual code.

1) Turn your ignition to II, but don't start the car.
2) Hold the "read" button on your turn signal stalk
BE QUICK HERE, timing is everything
3) Press the rear foglight switch twice.
4) Look at your trip comp.
5) Keep pressing "read"

After a short pause, the display (hopefully! ) reports:

No DTCs found.

Reported systems are:

BCM (Brake Control Module)
DEM (Differential Electronics Module - AWD)
ECM (Engine Control Module)
SWM (Steering Wheel Module) (probably only if DSTC)
EPS (Electrical Power Steering)
CEM (Central Electronic Module)
SRS (Supplemental Restraint System)
CCM (Climate Control Module)
DDM (Driver Door Module)
ICM (Infotainment Control Module)
PDM (Passenger Door Module)
KDM (Keyless drive module)

Enjoy
Stan