The Jag and Rover guys do it this way,
How to permanently disable the Eco stop start
How do I disable auto start/stop? - Jaguar Forums - Jaguar Enthusiasts Forum
Rear of your car,
Lift up the tail gate,
Lift out the boot lining ( there are two sections that just clip out )
You will see two battery's
Do not touch the big battery that's located on the left hand side.
Disconnect the earth terminal on the small battery that's located in the rear center ( the battery looks the same size as a pint of milk in a carton )
Cable tie the earth strap out of the way ( keep it away from the positive lead though!
The above procedure should take no longer than 5 mins.
Job done!
No more forgetting to press your Eco button
No warming lights on your dash,
No cutting out at junctions,
My C class coupe has big battery under hood and small the one in boot, will this work on Merc`s.