We'd been to the village of Rebeccu 18 months previously, just after visiting the nearby domus de janas Sant' Andrea Priu and not found this, but we had better instructions this time ...
Drive into the village and park in the main square. Exit the main square on foot opposite the way you came in, and follow the path which bends to the right, leaving the village, and Snow White and the 7 Dwarves (no kidding, a full set of gnomes adorn the house on the way) to your right, with open fields falling away to the left. Another path from the village joins from the right, and less than 100m later, there's a track to the left with a signpost "Fonte".
The path was being cleared for the summer, and we exchanged greetings with 4 groups of men, scythes in hand but all taking breaks, on the way. About 150m along, there's a tiny wooden bridge, and the font is up on the right.
An incredibly verdant setting! If not for the grass cutters, we'd have had to wade through chest high greenery to get here!
The Nuraghic font itself is a tholos style chamber; water flows along a groove through the entrance hatch (about 60cm square) in the basalt block frontage and zigzags along the entrance hallway (about 5m long) and drains away under a paving slab at the end.