1992 BMW 318 Battery System

Electrical problem
1992 BMW 318 4 cyl Automatic 127000 miles

Hello,

This is the first time for me driving BMW car, one of my friends faced a problem with his car battery as its discharged and asked me to help him by making my battery in parallel, but when I opened the engine cover, I found just the (+) and I didn't find the (-) ! How can I do the parallel in batteries in this case?


Best Regards,
Mohd4692