yeah return line in the middle of the rail almost pointing downwards . a knock sensor reads engine noise , its like a microphone for a engine and the ecu only reads a certain frequency which is believed to be level for when knock ( detonation ) accures . here is a read up . my XR6T sensor only cost $10 with plug and bolt , and the engine it came from looked very clean and looked after .

http://www.engineblox.com/?p=65