I've been asked a couple of times to repost my instructions for connecting
an aux battery in a Westfalia that already has the isolation relay. Here 'tis:
+-----------------

Here's what I did to connect an auxilliary battery in my 87 Westfalia, which
came with the relay installed. Bear in mind that I'm assuming you have a few
basic electrical skills, if not, get some help from someone who does. 

1. Disconnect your primary battery at the ground strap.

2. Disconnect the 2 red wires leading from the fuse box behind the drivers
seat from the relay terminals. These wires lead to your fridge and cabin light.
 
3. Connect these two wires to the + terminal on the aux battery.

4. Run an 8 or 10 ga wire from the + terminal on the aux battery to terminal
#30 on the relay. This terminal supplies current to charge the aux battery
when the van is running.

5. Connect a ground strap to the aux battery. 

6. Reconnect the ground strap on the primary battery.

That's it. With this configuration, the aux battery supplies cabin power
whenever your engine is not running. When your engine is running, the
alternator charges both batteries and supplies cabin power.

-Mark McCulley
'87 syncro Westfalia
'74 bus (revived once again)