The upside down Δ is the "ground" symbol. In the case of a tractor, it is electrically the same voltage as the metallic frame of the tractor. The battery in the tractor is likely installed such that the negative pole of the battery is connected to the frame, while the positive pole is the source which feeds all electrical appliances. Note that the battery is nominally 12V, but is actually ~12.5V with the engine stopped, and ~14V with the engine running and the charging system on. The negative pole of the battery, and the tractor frame is referred to as "ground", or 0V. The positive pole of the battery is usually referred to as "12V power bus"
When installing a new 12V powered appliance, like a radio, by default, the radio's case is electrically connected to the frame of the tractor through its mechanical mounting. This means that to supply power to the radio, only a single wire needs to be connected, and that wire would indirectly (through a switch and/or fuse) connect back to the 12V power bus.
I'm assuming that the sensor you installed is built in such that it is in an insulated case, so that no default connection to ground occurs when the case is mounted to the tractor frame, and it has three wires. If this is not so, tell us.
This means that to install the sensor, there are three wires to hook up, one that connects to the power bus through a suitable fuse, one that connects to ground (frame), and the sensor output, which connects to some external circuit.
The big question I tried to ask some time ago, is how did the maker of the sensor intend it to be connected? If you have a data sheet for the sensor, or a part number, can you post that for us to look at, and that might answer all these questions...