95 4runer drs lock but won't unlock

Thanks for your time. A couple of things to know about this 95 4Runner Limited.  The Factory keyless entry and alarm have been taken out and bypassed.  Power door locks is all  that we are dealing with.  Two or three years like this and not one problem.  Keeping that in mind, The doors will lock but won't unlock using the door switches and If you put the key in the door it will lock and unlock. How would I troubleshoot this?