need to know where the origional power cable comes from and follows through the car to the radio in my bmw e36

when i bought my MBW e36 coupe, it had all the right connections coming out of the hole where the radio would normally be except one wire, the power cable!! i need to know where the origional power cable origionates from and how it is threaded though the car!! i dont want to do a cowboy act and start cutting cables and rejoining them just so i can get power!! i want it to come from the one place it is supposed to come from and properly so that the radio will only come on when i turn the key, not so that i have to physically turn the radio off manually! hope you people out there can help