Aftermarket central locking, how to repair

A frustrating problem with these aftermarket central locking kits, after some weeks of daily use the doors would not lock 9 times out of 10 but they would un-lock. I traced it to faulty relays on the control PCB, decided to document how I fixed the problem in case anybody else has the same issue.
It's an easy fix, 2 months after I made this repair and the central locking is still going strong with no further issues. Hope this helps!