How do you trigger a Menvier SD1 dialler using an auxillary input?

I have a Menvier SD1 speech dialler which I want to trigger using just a 12v PSU and a relay from an auxillary system. I am trying to trigger it by switching the 0 volts of the PSU through the relay but its not working. I know im doing something stupid as im not an alarm engineer so I would be grateful for some help.
, thanks.