Greg,
We installed a "Yellow Top" Optima Battery under the driver's seat in our '86 Syncro Westfalia about four years ago. We added heavy wire to the starter battery complete with heavy fuse and relay. Works flawlessly.
-Larry

How many , Have installed a (Deep Cycle) Optima as Aux Battery in Westy's using the stock wiring under the drivers seat (silver or black cube about 1"x1"x1" with several electrical terminals) & with what results? Has anyone had good results? Has anyone had any problems?