L320 under bonnet shows a space on the RHS under the cover able to take a 096 battery( 72AH) (just) or 063 battery (45 AH ) with ease.
The 063 can just be dropped into the space available while the 096 requires some modification.
I opted for the 096 as I had one from a ford Kuga lease car I returned.
To get it in is tight ...you need to remove the clamp down plastic from the front with a standard wood saw


and you will find a plastic incline at the front of the battery space which you can angle grind out or put a piece of 1/2 in ply under the battery to lift it . The battery must not press on the brake reservoir for obvious reasons .
Two lengths of 5mm rod screw into holes in the base and a cross bar secures the battery.
Then you need the electronics . A Voltage sensitive relay is the simple way . Available for about £12 on e bay.



You can get ones with a LED to indicate operation or a built in voltmeter. On the led only types the LED
is very dim ,only visible from over head,the ones with a voltmeter indicate operation more clearly.
You can build a bracket from some aluminium to support the VSR behind the battery.






Finding an earth near the second battery is difficult so I chose to run a cable across to the main battery.At the battery ends of the live cables I used a 70 A fuse. There is a spare bolt hole (8mm) on the main battery live to make a connection.
Use thick wire rated 28A or better but dont get the idea you will get big charge currents ..about 15A and falling as the second battery comes up. Pass the cable just under the front edge of the scuttle ,loads of holes for cable ties. It may be a 150A alternator but that is to supply vehicle loads like wipers /fans etc.
I wanted the extra battery to power my fridge when on the ferry etc so a wire through to fuse47 will make the rear socket live 24/7 hence the roll of wire in the first photo . Yet to pass it through.

Top tip ...when its installed the wiring from starting to second battery it will be red and stand out so just spray with a can and it all blends in Always listen to old people or when they die you will live on in ignorance.
The avatar is a picture of the man with a big cock.
Learned to 4x4 on 100 mile beach and Frazer Island QLD
Dont spend money on old cars you will never get it back