C5 door lock switch L.H. side
 

My LH door lock botton, when pressed, will unlock both doors. but, does not work to lock either door when pressed. All other controls on the LH door, (window and mirror switches), work fine. All switches and buttons on the RH door work fine. The RH door lock button when pressed, will lock both doors. How can I tell if the problem is in the LH switch or in the black control box. I don't want to replace one only to find the problem is with the other. Both are expensive parts to replace. Any help or advice would be appreciated.Please don't tell me to go to a dealer. I cant afford it.
Chuck