Have experienced a couple of owners recently complaining of drained batteries and tracing it to the pre heat unit staying on which can flatten a battery overnight.
As a replacement they are expensive, hard to find and can be a pain in the rear to change so recommend my solution which it appears no one uses.
The power to the pre heat is supplied by the orange cable which if you cut it and put in a relay normally open (closed on power up) will prevent the discharge.
Connect the cables that operate the relay on to earth and the seconf to the top right 20A fuse in the under bonnet fuse box which only becomes live on switching on the ignition, and subsequently switches on the unit when you need it. Also by the fact that it is active on ignition being turned on the engine management system does not treat this as a fault
I did this mod 3 winters ago and never looked back
