I have one from a MY76 in front of me. They made a change from the earlier ones like your's by eliminating one relay. The one I have is p/n A 123 821 00 63. It won't work in the earlier years, however looking at the solder connections on the relay as well as the connections coming from the pins, they look to be possible problem areas. As well there are 2 electrolytic capacitors that are also suspect (47uF 16 volt).
You could check the relay coils and contacts with an ohmmeter. Coils should read just under 100 ohms.
The one I have here also has the component leads cut off very short so they could have cracks in the solder joints.

Have you opened the relay up to give it a visual inspection? Might just need some re-soldering.