OpenSky

It happened and you slept in your car. All other advice is moot at this point, the event happened. Full disclosure : I am not a lawyer but come from a family full of lawyers ( US based)

I assume you have the confirmation page with the reservation etc, guaranteed preferably with money or a credit card.
I assume also you can pull up Marriotts terms and conditions regarding guarantee etc.

And understandably, since you had to sleep in your car there were losses involved, i.e loss of work, billable hours etc, since you did not have the amenities of room, a desk to work on, a telephone ( call me old fashioned) and wired internet? Add all these up. If this was me the losses would be in the low thousands if it was a business trip, due to inability to work remotely.

Enough with the hotel blaming Marriott and Marriott blaming the hotel.

Go to the small claims court in appropriate jurisdiction
Sue both the hotel and Marriott, for losses due to their "negligence". Sue for the amount of your losses.
Name both the hotel and Marriott as defendants.

The room guarantee thing will fall into place later.At best you will walk away from that and some $.
They can't give you a room on the night you wanted it. That ship sailed.

Note this strategy won't work for the airlines, in the US, those are covered by the airline deregulation act. A whole other can of worms You may want to consult a CA lawyer, if you know of one.

I would also find out who regulates Hotels in California (if any) and file a complaint re: this incident also.

DO BOTH.
These folks need to learn a lesson. Speaking as someone who has personally been walked before.