I seem to recall there is a signal in the keyswitch that detects whether the
key is inserted.  It activates an electro-nag to keep you from leaving your
keys in the ignition.  You can use this to activate the relay for the
accessory power.  Then, the accessory power only works when the key is in
the ignition.  Doesn't have to be 'on' just 'in'.

Mark D. may have more information on this or query the archives.

Pensioner