Ignition switched feed without “implausible value” code?

[Jim Dieseldawg]

Hi folks,
How do I connect to an ignition-switched feed without getting “implausible value” or “implausible signal” error codes? C30 D5 SE, 2007/8 model year (it was registered late in 2007 so might be a 2008 model year).
The main power board is above the passenger footwell.
I need the ignition-switched feed for lighting aftermarket gauges and to trigger a relay for an Alpine horn. I’m considering running a relay-switched auxiliary fusebox for all aftermarket things but (a) I’ll still need an ignition-switched feed to trigger it and (b) it would be nice to light the oil pressure gauge and boost gauge with what’s already there so I can leave the horn project for later.
Any and all suggestions will be welcomed!

[IainG]

Hi
I use the 12v socket by the gear stick as a feed for my camera. It could make an ideal supply to switch a relay. The panel on the v50 just pulls out and easy to connect wires to the wiring on the back.