A generic (non-BMW) disconnect would be easy enough to whip up....and the disconnect switches are widely available as well. That being said though, it would be nice to have everything all ready to mount and cut-to-length (if that turns out to be the case with the OP's unit).

Factory type would be great but I wouldn't mount it in sight or to the cover over spare. I would have it accessible above the cover but not attached to it unless the cover was slotted so I could easily detach if I wanted to get the cover out.

The "Transport Mode" was introduced by BMW in 2002 production year. Depending on model, certain features (sensors) are turned off, and the rpm are limited to 2,000 or 3,000 so the dockworkers won't have a field day.
The "Transport Mode" is supposed to be turned off by dealer as soon as the cars hit the lot.

Also, there were instances where some cars (some e39, I don't know about other models) would go into "Transport Mode" when disconnecting the battery without waiting for those 16 minutes or so for the car to go in sleep mode. Probably those cars had already a weak battery (this is a royal PITA, because you can't turn it off if you don't have access to INPA or GT1.)

Here is an excerpt from the TIS: "To prevent batteries being subjected to excessive power draw during transportation from the factory to the dealership, the E65 was the first vehicle to be equipped with a so-called Transport Mode (since start of series production). ne Transport mode prevents various control units from being activated by the user. This prevents a number of different functions (e.g. radio, TV, interior lights) from being used while transport mode is active."

Also, in "Transport Mode", the car is locked, but all sensors (tilt, hood, etc) are disabled so if the car is jolted during transport either in a cargo container or on a tow truck, the alarm doesn't go off and subsequently drains the battery.

Are you talking about the explosive terminal disconnect that trips with the airbags if they deploy?

I think I would rather take the spare out than set off an air bag every time i want to disconnect the battery but can't be sure unless I try it--where can I get it?

no, no... not the safety terminal on the (+) endo of the battery... it is a device, aftermarket, that installs onto the battery (don't remember, (+) or (-)) and it will cut off the battery completely if the charge level drops below a specified threshhold. the device is connected to the battery, and somehow "gates" the juice flow, and when there is a legitimate demand for the power, such as lights come on, or the modules are awaken, it opens up the full battery potential...

i can't remember the name of the widget, or any other keywords, but it was kind of a smart cut off switch...

thank you for the explanation, it seems that is what i would need to activate before shipment of the car, assuming it is available on the 2006 E53... i saw it in the INPA but it does not mean that it would work...

TerminatorX5:
Given your description is this what you were thinking of:

12 Volt Battery Saver Auto Disconnect Switch

bingo!!!!

and here a page that was referenced a while ago...

BMW X5 - How to Install a PriorityStart

I want in on this quick disconnect. So cool, and so necessary for us DIYers.