2nd battery for T5 vw transporter

[sparx]

Can anyone help,
Need to add second (leisure) battery under passenger seat, main heavy wiring done and split charge relay in place but need to find 'pilot' wire controlled by ignition switch which is dead when cranking engine eg fan/cigar lighter/radio.
Preferably without dismantling dash, and wishfully one of the cores under the seats!!
Have bought CD of van systems but pretty useless as no locations/routes of wiring given.
thanks, sparx (industrial type)