Just installed second battery under the rear seat... (I.Is there a way to use the rear heater switch in the dash to disarm charging of the second battery while driving?) (II. While I'm on it: is there a way to cause the horn or any horn, or an alarm, to go off whenever A. the igition is off and B. the automatic 'shifter' is not in park?) . . . want to use the battery area under the driver seat for something useful: 

So maybe: a chemical dryer as sold at West Marine, a big tub which allows complete draining, a hole at the bottom of the battery compartment, so that water doesn't have to be emptied, but will drain to the exterior ground, a screen for the chemical dryer so it sits above the hole of the tub which said hole is connected to a hose, maybe even a fan under the driver seat, and vents at the rear, so that air is moved across the chemical dryer, which you don't want to spill anywhere, cause its a corrosive salt, (think its calcium chloride...?) . . . so evey now and then you slide back the driver seat, and add some more of this drying agent . . . and then forget about it . .  though maybe you want the drain hose postitioned so nothing ever splatters on your brakes et .. .. .