It's easy to do but I would leave it keyed on/off, my common sense tells me that even the smallest thing you might plug in and forget on would kill your battery. What's your reasoning for wanting it hot all the time?
J-n-M, I would like to use the back outlet for some LED lights for cargo area and general campsite lighting. The stock lights draw too much juice to leave on for long periods but it would take a long time for an LED to drain battery. I'd like to be able to shut off the ceiling switches but leave key off and back door open, with LED light.
Another option would be to wire up a switch from a constant hot source to send power to the relay when you want the outlets to be on but don't want to turn the key on.
I made two outlets hot all the time...
For fridge and any other equipment....
I do however have an aux battery for this....
Wire 6AW runs from battery to outlet fused at the battery...
I've got a dual battery in my Jeep. I've got 2 always on plugs at the back. One for my fridge and one for lights, charging etc. I've run them directly off the battery with a fuse.
I would like to do the same for a different reason. I want to be able to charge the battery with a solar charger (accessory outlet style) while camped for several days. Any advice would be appreciated.