Whether or not it was? Only you can actually know, because only you can answer the question: was I consenting to sex in my mind when we had sex?
A lot of what you've written points to you perhaps already know that getting in his car was not a great decision. We make bad decisions, it's the human condition.
Quoted for truth.
Most (if not all) rape accounts contain a moment where a choice was made. That's the choice rape victims beat themselves up about. It's the " if I hadn't" - as in,
if I hadn't (gotten in the car, gone to the store, smiled at them, etc), I wouldn't have been raped.
But that pivot moment doesn't make you responsible for someone raping you.
The time that matters is what is in your head proximal to the actual assault. Do you think you are deciding to be there, or not?
I've had a decent amount of regret sex, and I've been raped a few times. One rape in particular I was very....What? Co-operative? Tolerant? Something...definitely no fighting back. But it involved a weapon. And even though most of my memory of it was that I was compliant, I do remember that brief burst of a moment when I knew they had a thing that could kill me.
So, rape.
A regret-sex incident: A guy picked me up at a bar, I wasn't even close to drunk. Fast forward, he seemed to have a lot invested in being extra-dominant and rough. At one point I decided I was done, and made a move to get him off me, but I did not succeed. And I really did decide to just get it over with.
Some people would say that move was my 'no'. Some might say I wasn't able to clearly see what was happening to me because on a certain level, I was in a trauma-recreation loop.
But my personal bottom line is, I chose to let it happen. So I would not call it rape. I did place myself in the situation, I did have my faculties, I did make the beginning of a 'stop' action - but then I decided it was close enough to done. I was not afraid I wouldn't get out of there, and I wasn't afraid it would get worse.
In your situation - it's hard to tell. You made it clear to the guy that you weren't interested, you said. I'm curious - is it at all possible you were not definitive? I'm not saying that makes it rape or not - but it's the only tangible moment of consent/non-consent that is in the clear for you, memory-wise. So it might be helpful for you to pick that apart a little.