Hurr,

That's interesting I wonder if it's year dependant whether the +B cable runs into the rear seats?

My '91 525i had the main bus (IE starter motor capable) cable run straight under the rear seats with a mounting point for the battery lead...

Snap us a photo of under your rear seats and lets have a look see if yours has the point.

I'd have guessed that your original battery cable from under the bonnet/hood would bolt to this mounting point... but vadim_braia seems to have experience otherwise.

Also : Categorically YES you have to use a vent tube on any battery under the rear seat... unless you want explosive hydrogen inside your car. (The chances of an actual explosion are small, but not unheard of.)