Dummy lights are just either on or off thus controlled via grounding the circuit. I assume your light is staying on all the time when it's connected to the sensor.

First, make sure your instrument panel is grounded. Not sure how 69's are grounded but if it's not obvious then just manual ground with wire clips for testing purposes. If it still stays on then replace sensor. Sensors switches are cheaper than the gauged type.

Most electrical "odd behaviour" is due to bad grounding.