I assume, you have a table, where are registered, which cars all customers have (something like tblCustomerCars: CustomerCarID, CustomerID, CarID).
And I also assume, that the form you are designing, is based on cars table.
So instead of cascading combos, why don't you use a single one, linked to CarID, and as RowSoure having a query like
Code:
SELECT cc.CarID, cust.CustomerName & ": " & car.CarRegistration As CustomerCar FROM (tblCustomerCars cc INNER JOIN tCars car ON car.CarID = cc.CarID) INNER JOIN tCustomers cust ON cust.CustID = cc.CustID ORDER BY 2 ASC
When you click on combo, you'l have selections like "John Smith: XYZ 123", ordered by customers and then by cars of this customer. When you select any of selections, the combo will get the value of CarID of selected car. And no code is needed!