93 suburban rear doors

I just got a 93 GMC suburban. My rear doors do not unlock or lock with the other 4 doors (when i use the power locks). I was wondering if this is normal for this year or is there a problem. Any help? Thanks.

Simple, clean the contacts.. Open the passenger side door and you will see spring loaded contacts. Clean those and the ones on the door. 1 minute later, your good. Pat