The Mtroniks sound unit has two white wires which control its functions,
Engine Sound on/off (Toggle action, held at 0v for engine sounds)
Horn (Momentary action for short blast of horn, 0v)
You need an extra device to control these signals, Mtroniks sell one called the 'micro switcher' but I decided to build my own.
Components - PIC 12F683
8 pin DIL socket
small piece of veroboard
The PIC program is designed to (with reference to the transmitter stick) -
Stick moved fully down, toggle sound on, stick has to be returned to neutral and then back down to toggle sound off.
Stick moved fully up, trigger single horn blast, stick has to be returned to neutral and then back up before another blast can be generated.
This is a very simple design and easy to make on a small piece of stripboard.
Diagram viewed from component side e.g. above PIC