I certainly haven't seen them all, but every one I've been offered was a company contracted with GM, Ford, etc. Much like financing.
My son recently bought a truck. He had approved financing through his credit union. The dealer where he finally bought said they did not use outside financing. "Okay, if you can match or beat my CU rate, no problem." He walks away and returns in a few minutes, "We can get really close." Son gets up and thanks the guy for his time. "Oh, wait. Let me see what I can do." Uh, huh. Nice try.