Clipping onto the harness wires never works with standard lights. The only way i've been able to get an accurate reading with a timing light is by a wire between the coilpack and spark plug. (believe me, many of us have spent weeks figuring this out)
Try this writeup, it's pretty good and includes pics.
zerothread?id=59684btw, you don't necessarily need an extra coilpack, just gently stick the wire into the coil and the other end on the spark plug, you'll see what i mean.