[Hester]

Here's my $.02: (1) a battery cut-off switch mounted in reach of the driver from behind the wheel is a good safety feature. If its not real obvious, its even better (for security). Unless you use a relay and solenoid actuated switch, mounting a battery cut-off in the passenger compartment will mean running battery cables to and from the switch because the switch will be completing/interrupting the circuit between the positive side of the batteries and the engine compartment. (2) While a battery cut-off switch is a good thing to have, installing one to compensate for a battery drain is overkill. Just find the circuit that is staying hot when the ignition is off. That "professionally installed" radar detector is a good suspect. Ideally, something like that should be wired to a circuit that is hot only when the ignition is on. The method that Rusty recommended is a good one. Another way is to use a 12 volt test light to determine which fuses are "hot" on both sides of the circuit when the ignition is off (make sure the interior lights are off).