You must have a California car as the mounted light only appeared on 93-95 Federal models.
Turn the key on engine OFF and depress the button for 2-4 seconds. The light will be lit as you press the button and go out as you release. The control unit will cause the light to blink out a fault code. One blink indicates no codes present. If there is more than one blink count them. This will be the first code. Repeat the code retrieval by again depressing the button and count the blinks. If you get the same number then there is only one code. Keep repeating till all codes are read. This is indicated by the repeat of the first code received.
Bosch Master, ASE Master, L1
33 years MB technician