I have a 1994 S-10 Blazer with the CPI 4.3 V6 and I am installing a tachometer on it. The instructions for the tach say to connect it to the negative side of the ignition coil, but the coil on the Blazer has 4 red and white wires going to it and I can't tell which is negative. Please help if you know which wire is the negative.