I have an image for you to look at, and I will need confirmation that this is or is or is not what you are giving reference to.
As far as your issue goes, cable P7 should be marked on both the connector block, and quite possibly onto the motherboard. How many pins are on the connector? if its 4 (two rows of two) then this will plug into the motherboard, and is the Intel board supply. If it is either a 20 or 24 pin block, then this will plug into the motherboard, and this supplies your entire mainboard components with power. If it is 4 or 5 pinned, has red, yellow, and black wiring, and has two corners cut off at one side, then this is the power supply cable for a hard disk drive or CD/DVD drive.
If it is a 4 or five pins white or black block spade like connector, then this will serve a floppy, flop drive for power. If it as spade type connector with loads of pins on it, and about the same size as the hard drive and CD/DVD drive power cable as above described, then this is a SATA power cable, and will power any SATA devices. You will also notice that a SATA drive will also have a smaller complete red or black cable that is similar to its bigger SATA power supply cable. The smaller one is for data.
all of these power cables have grooves, notches or angles cut on either one side, or are shaped so that they will only fit one way. Do not try to force these into any other slot, or you may cause damage.
I hope that this is the fixya for you, and if in the meantime you have any further queries or comments, please do not hesitate to get in contact with me.