installed shift light with rpm activated switch now no tach reading help!!!

i installed a shift light in my 93 ex coupe with a b16 and i used a rpm activated switch from msd. i hooked it up correctly and i used the blue wire behind the gauge cluster to get the tach reading but now the tach doesnt work, i disconnected the wire from the rpm wire on the rpm switch to the tach and the tach works again. i then tried taping in to the negative side of the coil through a wire coming off of the distributor, but then again the tach stopped working. does anyone have any experience with this? do you think that if i put a diode in line from the tach to the rpm activated swith that it would work. any help would be appreciated thanks.