hi mate
you need to finsd something that comes on only with ignition really.
does a disco 2 have a fuel pump? if so, that could be good.
what you really want is something which goes live when the engine is running. electric fuel pump, or diesel shut off solenoid would be ideal.
how does the alternator light work on a d2 then?
personally i would work hard to find something that is live ony when the engine is running, otherwise the split charge kinda defeats the object!!