Great question!
Since the signals from the two Voyager spacecraft are SO faint at their current distances (e.g., Voyager 1 is an entire light-day from Earth), if we did not have a very good idea where they are, we would be unlikely to ever find them.
As to the instruments used to communicate with them, we rely on the NASA Deep Space Network, supplemented occasionally by the Very Large Array, the Very Long Baseline Array, the European Deep Space Network and the Japanese Deep Space Network.
Here is a rough diagram of the NASA DSN:
and here are pictures of the radiotelescopes at each location:
We are currently in the process of upgrading the dishes at each location: