... you hold your ground, and in front of them prepare to start dialing their regional center (after you ask them for the number), informing the counter people that you will work your way up to corporate if needed, and let them know to "not go anywhere" because you may need them to speak on the phone!
They immediately found me without my needing to initiate the call - a sales manager's personal vehicle! I kid you not!
This took place a few years ago in an Enterprise location within a Nissan dealership, located in the upper Midwest.
I've had great success with this technique in a myriad of business environments. I simply request a manager, and when the manager claims he cannot help me, I non-antagonistically but firmly let them know I need to escalate with their regional manager, and I kindly ask them for their regional manager's direct number.
If they're reticent to give-out the manager's number ("it's against our policy"), I get a little firmer, saying:
"I believe you're trying your best here, and when you give me the number I'm going to say exactly that to your manager, because I believe the problem may be with the system you're in rather than you personally. But if you refuse to give me the number so I can work this out with them, I assure you one-way-or-another I will get their number and subsequently let them know you're attempting to impede me from speaking to them"
"So what's it going to be? I'd like that number."
With that above, I've literally never had an instance where something to my satisfaction has not been done at the branch store level, and I've never had to make the call. When finally satisfied, I kindly ask the person assisting me:
"Who can I call or email to make you look good, since you went way out of your way for me? I'd like to thank you."
If they give me a contact (they always do), I do my best to do a very quick follow-up praising the individual (s) by name for finding a solution. I may need these people again, and I want them on my side - and besides, it's the right thing to do. I have turned bad situations into good like this, and have established seemingly good repeat customer relationships despite a rocky initial interaction.