use your multi metre, place a cut wire on the serial cable port and check inside to see the numbers (i.e 5,4,3,2,1 etc) try to fit the tested wire on each pin slot and read it with the corresponding colours from the other end of the open cable. monitor the cable while reading it and this will allow you to identify the colour code… the colour code numbers needed are (5,3,2). If you are able to get the respective colour code numbers, you can no use your rj45 connector and fix it in the first 1,2,3 section of the connector… this should do..
So in simple language.. 5 3 2 (serial pin slot) to 1 2 3 (rj45 connector).. If you get it let me know.. knowledge is free