Haven't seen this mentioned, so I'll mention it.

Recently acquired an EA Fairmont wagon. It has an electric lock for the tailgate, operated by a switch on the dash.

I fitted a Jaycar remote central locking kit. When it locks, the green wire goes positive for about one second. By connecting a diode from this green wire to the brown/red wire coming from the tailgate lock switch, the tailgate will lock when the doors lock. No more forgetting to lock the tailgate.

Any common little black power diode will do. 1N4001 for example. Kathode (the end with the stripe) goes to the brown/red wire from the switch. The yellow wire going to the center of the tailgate switch is a good place to get power for the central locking unit.

You can hook it up to the RH rear door actuator.
With the EF/EL wagons, you can replace the tailgate actuator with an EA/EB/ED wagon one, as the Jaycar kits only have 4 actuators.