the Hella 87181 battery disconnect was installed by the original smart center Portland that was run by Kuni Auto Group. Their smart car service team still services smart cars at http://www.kuniautocenterportland.com/
The battery disconnect switch is connected to the negative battery terminal.
for spare battery disconnect keys, google: Hella 87185
Their former Service Manager, Bryce Moilanen, would tell you in the strongest of terms that Automatic Disconnect products (like the one offered by Roadmaster) must not be used in a smart car. People have done things as simple as entering their smart to retrieve an item (while in tow mode) and accidentally turning the ignition key to the ON position without realizing it. The computer then woke and engaged systems that caused damage beyond repair while being towed.
The computer does not know that the car is being towed. It only understands that it is moving and the engine isn't running. It does not have a 'tow mode' option programmed into it.
For those of you successfully not using any battery disconnect - you are the exception. I will not risk it. I made a large-print copy of the flat towing instructions from the owners manual, laminated it, and follow it religiously.