Keith L said:
Many thanks for the feed back; however, I would still need the 12v ign feed to power the relay, surely?
1. No, splice into the 12v supply to the existing rear socket behind the panel using both cores of a 2 core 10 amp cable.
2. Split the other end of the cable and connect to the relay terminals 86 & 30.
3. Connect the relay terminal 85 to an earth using 1 core 10 amp cable.
4. Connect the relay terminal 87 to the extra sockets power supply terminal, using 1 core 10 amp cable with a 10 amp inline fuse on the cable.
5. Connect the extra sockets earth terminal to an earth.
NOTE: Forget any other terminals that might be on the relay, but it must be a 14v not a 12v relay.
Example of relay:
http://www.ebay.co.uk/itm/Low-Current-Switching-Relay-HG4185A-012-1H-14V-DC-40A-/150944238093?pt=LH_DefaultDomain_3&hash=item2324fa4a0d
This will leave your existing socket working as normal, but when you start the engine and the alternator kicks in the voltage will increase to 14.2v and more. This will switch on the relay, which in turn will make the extra socket live.
When you turn off the engine the voltage will return to approx 12v, the relay will switch off and the extra socket will be dead.
Hope this makes things a bit clearer