set up TruckID is an auto number and is only used by Access, then use TruckModelID as a short-text field.  Then have CustomerID as a relationship field in your truck table.  Then in Relationship window, Join CustomerID between the two tables.

Oops:  CustomerID should be a long-number field in the Truck table that is only included as a joining or relationship field.  It is hard to type with one finger in a brace :)

Thank you Julie, I will try that.
I still have a lingering question though.
My CustomerT table has both a CustomerID and a TruckID field. lets say Customer 1 has 20 Trucks total. 10 trucks are slightly older than the 10 other ones, meaning that some of the filter part numbers are different that the other 10.  I have made a MasterPartT for all filters any truck will use. How would I set my database up to account for this? 1 to many or many to 1 for the realtionship? I just want to enter a TruckID and it return all the filters that truck uses. It Could be as many as 10 different filters, depending on the truck.
Thank you.

Customer table to Truck table is one to many, Truck table to Parts Table would be many to many as more than one customer will have the same make/model of truck.  If you have a look at Richard's free videos on how to set up Customers with orders with order details, it will give you ideas on how to set up your Customer-Truck-Parts database.  These helped me by giving me ideas on how to look at the problem from my perspective.  Often I found I had missed a step in my mental evaluation of the database.  I also found Richard's idea of using Excel to draw up a outline or plan of what I wanted to achieve; I often found I had missed a clue and had to add another field to my table or do another query or two to get to my end result.  If your queries aren't working, then figuring out relationships is necessary, otherwise build your tables first then worry about relationships when building queries.
You are on the right track.

David, I've drawn up a quick outline in Excel of how I see your database (image attached). I hope this helps.  I use colour to show the links between tables and gave a brief idea of queries needed to join them

Thank you Julie.