Ignition on auto locking

Hi, I have a Selca alarm. It has a blue wire which is meant to be attached to the odometer, so that when the vehicle starts to move it locks all the doors. The instructions do not indicate whether it is a positive or negative input. So I turned the ignition on and touched the blue wire to a chasis ground...nothing happened, but when I did it again it locked all the doors. As I cannot easily gain access to the odometer or speedometer and I have a feeling they are mechanical rather than electronic. I tried to simulate it with a relay so the positive when ignition on, gets turned to a ground output, but this did not work. Anyone have any ideas?????? Thanks, Mo

Hi, it's a Vaxhall Midi Van (Also produced by Bedford and Isuzu). Thanks, Mo

That should be VAUXHALL Midi Van.

Production for Vauxhall Midi Van ended in the UK in 1995. I have a 1991 model. I doubt it produces a vss signal. So I need a circuit to simulate a ground pulsed VSS signal. Thanks Mo