Foreign Key normalization question

I'm building an app that has a customers table, a locations table, a
products table, and a product_locations table.

They make a diamond shape.

The locations table and the products table each have a customer_id
column that links back to the customers table.

Then the product_locations table table has just two columns: a
location_id column and a product_id column, each linking back to the
appropriate table.

I want to write a constraint or a trigger or something else that makes
sure that before a (location_id, product_id) tuple is inserted into the
product_locations table, the system verifies that the product links to
the same customer as the location.

How do I do this?

Thanks in advance.

Matt