If you just want to be able to run a compressor I wouldn't bother running directly from the battery, or messing with the junction box in the boot, just tap into the towing electrics circuit. This is already pre-wired into the DS (something the dealer doesn't want you to know so they can charge you a fortune to wire it for towing) and will either terminate at the tow socket if you have a tow bar fitted, or in a connector in the boot if you don't.
This provides both a 15A ignition switched feed and a 15A permanently live feed. Not as high current capacity as an Anderson plug which is usually 30A or 50A, but plenty for a 12v compressor if that's all you want to run off it.
The wiring diagram below will help you identify the right wires, but usual warning applies - don't mess with this stuff unless you know what you're doing, get an auto electrician to do it for you instead. The permanently live wire is just that - a permanently live connection direct to the battery!
DS Trailer Wiring.jpg
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