Blue connector under bonnet.
 

Am I right in saying the reason I'm getting 12V only when the ignition is off and the engine is not running is something to do with the ST SIGN fuse removal for the PnP?

no. has nothing to do with it.

So why on earth is the connector going dead when the engines running?

your ignition switch is dead or a blown fuse.

the current goes straight from the ignition switch to the 30amp "heater" fuse, to the connector.


http://www.boostedmiata.com/technica..._connector.jpg

Right, well I took that off and had a play with it (its more like a trip switch on a house, you just press it to reset it).

Now seems to be working fine. I was using that as the 12V feed for my EBC, and when I got my boost duty cycle up to 70 and the boost still wasnt rising i was sure there was something wrong!

Still strange I had 12V with the ignition and not the engine running, but at least it works now.

Thanks You