1 step: I add a new field in "Custom fields" with this options:
Field Type: Check Box; Required?: NO; Read Only?: NO; Publish: YES;
2 step: I go in section Vehicles->Vehicles
before section "Vehicle options (check any that apply)" there is the new field added
The problems are:
1) the new field is always checked and it's impossible to remove the check (backend)
2) in frontend the new field is always visible and appear with the reference "true" or "false"