Who is the customer?

Very often I hear one should have a customer perspective when developing software. Some agile methods propose working close with the customer.

But who is the customer? Is it the person paying for the development?

The end-customer is usually not difficult to identify for consumer products, which is what I work with since I'm in the car business. But even for such a closely related product as heavy trucks it is not obvious who is the customer. Is it the user of the truck (e.g. the driver) or the company buying the truck?

And looking inside the developing organisation it gets even more unclear. I work with developing architectures. Who is the customer of an architecture? Is it the end-customer? I think he could not care less if the car has an architecture or not, as long as it has the features and properties he wants.

Sven Grahn, former scientific director of the Swedish Space Corporation stated the best "test" of identifying the customer I have heard so far (freely translated from Swedish and maybe so distorted by memory Sven does not recognise it):

The customer is the person, or group, that when you remove them the activity (or product) becomes meaningless.

So for an in-vehicle architecture the primary customer of the architecture are the developers. If there would be no developers who who look at the architecture? (The end customer is also a customer, if he's not there what is the point of developing the car in the first place?)

Can I use this test to identify who is the customer of research? In most cases it is not the ones who sponsor it (e.g. government, foundations. etc).