1999 Porsche 911 Car won't turn over, battery drains...

I tried jumping starting to no avail, just clicks. I pulled the battery and it charged up quickly, still just clicks. I then pulled the battery and connected the jumper cables to the battery cables in the car and same thing. I noticed with my voltage meter on that the voltage on the meter went from 14.1 to 9.9 and jumped all over. lights came on in the car and dash lites too but wouldn't turn over and just clicked.

Could this be the voltage regulator and if so, where is it in this model? Is it easy to replace yourself?

Thanks!