adding a look up field based on info i enter

[pam0566]

In my database, there is a form frmvehicles. i would like the zip code field to populate based on teh DealerNo i select [in other words, when i select a dealer/ that dealer is linked to my forms, and would like the dealer's zip code to automatically populate. i have been playing with this for a while but no luck. HELP!/ i am uploading my database... I hope someone can help me figure this out.... i would like it to be automatic.. so i can query distances using this field at a later time. thanks!!! i shoudl say this is an .accdb data base.. if that matters.