the one wire, I think it is black. The coil packs have 3 wires in the same loom so if you don't trim back the tape/loom you wont get a correct reading. The reason it probably worked on your friends car is probably because it had a distributor with plug wires rather than coil on plugs like the Z.
Or as na300zx said on the PTU wire works (thats the reccomended location in FSM) also but I had mixed results (Multiple readings) from that location.