Hi Stanley,
Yes, your truck is ETV214 - 2008. I have spent for an hour to check in the drawing but I couldn't find out this function. It seems to be they don't design this function in this kind of truck.
The traction direction is controlled by AC and Canbus technology. When the truck come backward the only thing need to press the direction key then the canbus signal will send to controller and direct the controller to change the phase of the AC motor to make to truck change direction (not via direction contactor as the old model). Therefore, I see no way to get the signal to active the beeping alarm instruments automatically as you expected. You only can do it by manually. I do hope some experts on our forum can help you more.
Regards
Nuna