You could run an extra wire from the serial shield to the cage. Or use "my solution".
To prevent the wires to get pulled out of the docking connector add a tie-rap together with the serial cable to a steady part of your dashboard. I actually have all wires from the cage tie-rapped to the dashboard.

Edwin de Vaan
mk2 rev.7 Trillian # 080000263 6Gb blue/red