I read about many solutions for having a non keyed power socket in the rear of the Disco II's, however I have a simple fix that is quick and works quite well and has not been mentioned. Remove the 25 amp fuse for the circuit. Run a jumper wire from any constant on fused circuit  ( I used the one for the Clock) to the right hand side blade tab of the circuit that held the 25 amp accessory socket fuse. Done. Voila you have a fused circuit that is constantly on for your existing rear accessory socket. For additional safety you can use an inline fuse for your jumper.