redmine_qbo_vehicles/app/views/customer_appointments/_show_hook.html.erb

<% if appointment.customer.present? %>
  <% unless appointment.vehicle.present? &&%>
    <p>
    <%= link_to new_vehicle_path(customer_id: appointment.customer_id), class: 'icon icon-add' do %>
      <%= l(:label_add_vehicle) %>
    <% end %>
    </p>  
  <% end %>
<% end %>