I have just done it back in November so my memory is still fresh. I have an MD2-55 with the same problem afflicting every Volvo Penta hour counter, but I suppose that your alternator will be similar.
I found the right wire simply probing the voltage on the alternator using a tester with engine on and off. There is one small wire output that will go to +12V only when the engine is running, the large red wire will be always at 12V when the battery isolator is on. To the back of the meter you will feed the alternator +12V and the negative that you can bridge from any other instrument nearby since the Amps used by the counter are negligible. This way the counter will tick only when the engine is running. In my case the output is on a post on the top back of the alternator where there is one small red wire connected.
If you could post the picture of the wiring on your alternator I might be able to point it for you.
