the 7th wire is the shield, if you do not plan to use it as a serial mouse later then you will only need to find the ps/2 data and clock, without reverse engineering i would guess that clock could well be the top black and data the white one right under it, as it goes through a fair bit of circuitry that, to me, looks to be there for level switching, although it could also be the serial TX line.
if you have a scope you could try to power the mouse (given that you found the power pins) and look at the signals, if that white line is TX you may see a 12V signal, you will not get anything out of the ps/2 lines as it will need a clock on the clock pin to start doing anything, if you also have a clock generator then it would allow you to find out pretty much all the pins.
(as long as power and ground are correct, putting data on clock and clock on data will not hurt anything, the only problem i do see is putting the mouse TX (12V serial from the mouse) to one of the ps/2 pins (meant to be 5V) of the computer that you plan to test with, it should be protected, but could well not be and fry something)
