20 years ago I was dating a stripper for a While, she loved car sex. In fact we did it more often in my car or outdoors than anywhere else combined. She was an utter lunatic anyway so nothing surprised me after a while..
There is an awesome beach in Ku Ring Gai NP that is a bit of a bush walk to get to but well worth it for seclusion. Took a girl there once and we only shared the beach with a big arse goanna and a couple of rock wallabies. She was a bit concerned the boats could see us so we went behind a big rock...
So to the question, car parks where people just don't go at night, like secluded beaches or parks for example. It is getting harder if this is your cup of tea, personally unless it's a spontaneous quickie because the lady I was with and I were just too damn horny then I prefer it indoors, so get a room.