Warning horn not getting ground

[Jeanric]

I have a 1994 OMC Cobra Ford 5.0 Engine. The warning horn does not work, whether when I turn the ignition on, or when I simulate a failure by grounding the oil pressure switch.
It gets +12V through the purple wire, but not -12V through the tan wire. Where should I look next?

 

[jimmbo]

Try grounding the Wire that Goes to the Pressure Switch, with the Ignition on.
Of course the Horn, might for be an Overheat

 

[Scott Danforth]

make sure you are grounding the correct wire. If I recall, there are 2 connectors between the temp switch and the main harness. make sure you have continuity.