The original BMW style battery is the best option for you vehicle, I believe it is a 720 CCA rated battery, if not Douglas is the company that was making batteries for BMW at that time so you can try to locate a Douglas dealer.
Its on the track of the car on the right side wall under the gray carpet. I mean TRUNK sorry :)
How can i install a starter on a 2002 polaris 325 magaum
trunk
In the trunk
325
12v
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Your battery is in the trunk most likely, on the passengers side underneath your small tool carrier.
under the back seats. you have to lift the seats.
In the Boot! right hand side...
325
325 stock