dependent look up from diferent tables

hi guys i have a question, say for example i have several DB's ( DB's 1 = ford// DB 2 = volkswagen, now in each DB i have cars. )



and i have a a form that is a dropdown with labels, ie ford and volkswagen.
so when i select say ford i get a dropdown from DB1 or DB2 depending on selection.


how can i do that?