How to use a choiceField declared in the model, in a form. django

I have this in my model.py

class marca(models.Model):
    marcas = (
        ('chevrolet', 'Chevrolet'),
        ('mazda', 'Mazda'),
        ('nissan', 'Nissan'),
        ('toyota', 'Toyota'),
        ('mitsubishi', 'Mitsubishi'),
    )

    marca = models.CharField(max_length=2, choices= marcas)
    def __unicode__(self):
        return self.marca

And I need to use it in my form.py I tried this but it doesn't work.

class addVehiculoForm(forms.Form):
    placa                   = forms.CharField(widget = forms.TextInput())
    tipo                    = forms.CharField(max_length=2, widget=forms.Select(choices= tipos_vehiculo))
    marca                   = forms.CharField(max_length=2, widget=forms.Select(choices= marcas))