Finding a switched 12v thats also live when cranking starter

[Zebedee]

I'm trying to find a switched 12V live connection for the changeover switch on my LPG conversion but it says it also needs to be at 12v when cranking and i'm finding that most of the circuits i find cut out when the switch is turned to the start position (x-relay doing its thing to cut down the electrical system load so the starter gets more juice).
Any idea which circuits i should be looking for?

I've not got a haynes manual just the bentley one which doesn't cover carb'd engines though i'm not sure if that would make a difference.

Its on a 1988 1.9wbx engined caravelle.

[bigherb]

Terminal G2 on the back of the fusebox which goes through Fuse 18 should do it. Or an unfused circuit would be any of the plain black wires from the ignition switch.

[Zebedee]

Cheers.
I really hate the newer style of fusebox with all the bloody multiplugs and no access to the terminals I much prefer being able to get a multimeter on the actual fuses, it made things so much simpler.