The use of the verb "need" in progressive constructions is only marginally acceptable, and then only in some dialects. You'll hear it quite often in the Scottish variety of English, as in Ay, I daresay you'll be needing a good breakfast afore ye go!
But when it comes to Standard English, you are right that "The customer needs a wiring diagram" is the correct form.
The use of the verb "need" in progressive constructions is only marginally acceptable, and then only in some dialects. You'll hear it quite often in the Scottish variety of English, as in Ay, I daresay you'll be needing a good breakfast afore ye go!
But when it comes to Standard English, you are right that "The customer needs a wiring diagram" is the correct form.