autopage 225rf horn honks every 10mins

I have gotten 3 different calls from customers about their horn honking about every 10 minutes to 30 minutes. They have not been able to bring them to me yet. I am impatient and like to solve problems not avoid them. These just started in the last couple months, but of course the customer said it started right away.

One is a plymoth neon, toyota truck, and dodge truck. I used the negative out directly to the horn negative trigger. I usually multi test to find the right horn wire. Then i test it with a test light to see if the horn honks through the test ligh. Works very well this way. Just wondering if I need a diode in between the horn circuit

I am pretty sure they would not have 3 grounding issues but its possible.