Has anyone used a 3rd party to assist in the purchase of a new vehicle?

Yes! I've worked with Heidi several times: amazingautowoman.com. She takes a fixed fee (I forget, but $350-ish?), negotiates for you, and has a network of dealers that she works with. She has saved me time, and real dollars too (by finding out-of-state dealers who will ship me the car). One downside: she doesn't work with all brands. If you reach out to her, please tell her I recommended her. Good luck!

Tom - I would suggest going through Costco auto for best price and zero negotiation. Basically you go through Costco auto site (or you can call them I believe) - and put in the specs for the type of car you want. They match you to dealers that have the car and tell you the price. You show up at the dealer with your Costco card - and they give you the car at the price. I've purchased 3 autos this way - and was seamless. Only negotiation is around extended warranty and if you want to finance via the dealer. Was great experience.

I tried Costco auto too. Nice service. You still have to go to the dealer for paperwork & pickup. Prices were prenegotiated. I would've used it but somehow I extracted a really low offer from another dealer. Ended up being $500 cheaper than the Costco price.