Yea I found a kit through Crown part# 132702-005. Mounts on the bottom of the frame. The plug is in that general area. Crown didn't want to give me the code so I went into the distribution box and had to remove the forward alarm jumper J2 so it would only peep in reverse

Kit 132702-005
It's underneath drive motor on truck frame, kit comes with instructions, and turned on through features menu.i think
Available through you're local crown dealer.

some of these newer AC drive units use low voltage to switch circuits in a logic manger module that in turn controls the motor controller and contactors.
You could simply have connected to too low a voltage source.
I'd be willing to bet that truck has a plug for a programmable alarm somewhere to hook the horn to and that can be setup how to operate through the dash manager.

had this before on a 48volt hyster, drove me crazy becuase there was no problems with the unit.

check what your battery voltage is dropping to while accelerating.
i bet you installed a multi-voltage back up alarm and it see's the voltage drop to threshold and changes state to 36v from 48v or 24v from 36v.

my fix was install an alarm just for 48volts, let me know how you make out.

It's a FC 4520-50