Mel- if there are no lights, no action at all, on the VMM- first check for power. On the right side of the VMM there are two connectors. One has 23 pins, the other 35. On the large connector, have the tech verify there are power connections to the following pins:
24, 25, 26: +Batt volts (+12.2-13.2V)
Without these signals at proper levels, i.e. good voltage & good ground (I think) the VMM is dead. Here is a thread
where bad (low but not zero) voltage on pin26 left the front VMM "dead" or without LED's reporting activity. The wire to pin26 on my rear VMM is red, the others are white. The pin# positions are molded into the wire end clip on the connector for easy ID.