To make the script work for your model. You need to have these following nodes:
- scoop (bed)
- misc_a (where to car attach to)
- misc_b (where the winch rope coming from)
- misc_e (where the control panel is)
- misc_f (where the 2nd control panel is (optional))
* misc_a & misc_b's parent is set to scoop.

Create a xml file, paste the content below:

<?xml version="1.0" encoding="utf-8"?>
<FlatbedData xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <Model>your_model</Model>
    <AttachDummy>misc_a</AttachDummy>
    <WinchDummy>misc_b</WinchDummy>
    <ControlDummy>misc_e</ControlDummy>
    <ControlDummy2>misc_f</ControlDummy2>
    <ControlIsOutside>true</ControlIsOutside>
</FlatbedData>

Place it on GTAV\scripts\Flatbed\Vehicles

That's it.