I might be wrong, but pin #18 in the top relay does not emit any signal. Pin 18 in the relay is a cut off so that when it receives a signal it cuts power to the convertible top motors to prevent damage. Therefore, I would assume it gets the signal from the ECU for the speedometer. Wouldn't make sense for it to come from the Tachometer b/c you can rev the engine as much as you want when stationary and still raise/lower the top. Check the electrical diagrams in the back of a Bentley manual to verify.
Good luck
