audiovox ax900 in a 96 accord

Sup fellas, I got a lil' bit of a prob. I'm installing an audiovox ax900 in a 96 honda accord V6 and the flippin' thing won't learn the tach. I'm used to putting in dei's and nordic/pro starters, so the audiovox is pretty much a first time for me. I hooked up the PURPLE / white (tach) wire to the blue wire tach diagnostic plug in hondas which gives me the a/c voltage needed, but I keep getting 7 flashes which according to avox means that the tach isn't learned. The book is tellin' me that the tach wire is supposed to be hooked up to the negative side of the ign. coil. Do I hook it up to one of the coil wires that drops negative in voltage or is there a better spot that gives me a stronger voltage in a V6 accord? Thanks in advance.

Thanks for the reply bobk; I've hooked up the blue wire and I get the 7 flashes, thought it could be something else and rechecked my connections. Why would the manual say that the tach wire needs to see a negative from the coil?