I do not have an Autometer light but rather an MSD so I realize different companies are gonna have different schematics for their products. But for comparison: my light's RPM input wire goes to the coil's negative terminal for signal. If you have an MSD ignition, it will have a tach output connector on it.
*2012 Audi A3 TDI, Premium Plus, DSG, Ibis white
*1994 Pontiac Firebird Formula, Gt47-80 turbo, 580 rwhp/700 rwtq
*2004 Jeep Liberty Sport 4X4