Your going back a lot of years.

That handle should have 2 buttons one for the horn and one for the function.

Both are exactly the same except one has a orange cap the other a blue cap.

Both have 4 prongs for the wires in the back of the switch a normally open and a normally closed. Part numbers 115972-001 orange cap, 115972-002 blue cap.

These will pull right out of the handle be careful of the wires they run through the handle to the switches and are very thin. Might need a set of pliers to pull it out.

Pull the horn switch out just enough to test where the wires are (should be on the normally open prongs of the switch) In other words you should have battery volts on one wire of the switch and none on the other wire until the switch is pressed.

Not sure how long you had this lift but it sounds like the wires are in the wrong location ,or shorted. It sounds like the switch is working just backwards.

Here is a pick of the switch see the 4 prongs.
https://scottlift.ca/products/cr115972-001

EDIT I have no affiliation with that web site and not recommending you buy from them.