Fill existing data in form or add new to Access 2007 table if does not exist

[newbieX]

I have a relational database of cars (an example. My database is sensitive so I cannot use it but this example lays out the overall structure). The master table has basic information about the car. ID is unique and cannot contain duplicates.

Table: MakeModel
ID*          Model         Make          Model2
HAS          Accord        Honda         Sedan
HAC          Accord        Honda         Coupe
HCS          Civic         Honda         Sedan
HCC          Civic         Honda         Coupe
HCH          Civic         Honda         Hybrid
TCLE         Camry         Toyota        LE
TCHLE        Camry         Toyota        Hybrid LE

The user has a paper data entry form that he fills out when making a sale. I am creating a data entry form to populate the database after the data is handed to the data entry person. Because the form has over 100 possible fields, I have spilt the results into 2 separate tables so I can more easily manage the data. These tables have a one-to-one relationship with each other. They have a one-to-many relationship to the MakeModel table. ID and DateTime are primary keys for these two tables. Below are the field names for these two tables.

Table: CarData
ID*
DateTime*
SerialNumber
Color
Milage

Table: BuyerData
ID*
DateTime*
First


Last

My data entry form has all these fields in it. The problem is I am not sure how to write code to check to see if the ID exists in the MakeModel table, and if so, populate the fields ID, Model, Make, Model2 in the form with the data that exists in the table, otherwise add the new ID, Make, Model and Model2 to the MakeModel table. Right now I just get an error message telling me duplicate data exists. I have no code to include because I am totally new to this and do not even know where to begin. Any and all pointers are greatly appreciated.

[June7]

Database is so sensitive can't even use the real field names?

Apparently, breaking the table into two parts is not easier to manage. If it is truly a one-to-one relationship, then keep it as one table unless you will exceed 255 fields. If you do, then I suspect the real issue is data normalization. Even 100 fields is a lot and makes me wonder if it is normalized.