You are going to need the following structure for this:
Customer Table
Customer Locations Table (new)
Projects Table
Part Numbers Table
Relationships:
Customers < Locations (new)
Customers < Projects
Projects < Part Numbers
Locations < Part Numbers (new)
Add a Lookup field in the Projects<Part Numbers relationship to pass down the [Related Customer] field value from the Project into the Part Numbers table. Then, in the [Related Location] field, you will make it conditional so that the [Related Customer Record ID#] field value = [Related Customer] in the Locations table; to ensure that only Locations for the Customer assigned to that Project will display in the drop down when adding Part Numbers.
