If I leave a tank of new e-juice in the car and its hot outside it seems to taste better by the end of the day. Is there a difference in steeping and rushing the process by leaving it in the hot sun?
Steeping is a complex process that works differently for different juices. Partly it is oxidation as evidenced by the darkening of the liquid and the fact that larger full bottles steep much slower than smaller ones with more air space per ml. It's also a matter of the flavour molecules combining with the PG/VG/nicotine and each other.
Sunlight and heat are generally going to be a bad thing in large quantities so both are probably best avoided. PG is more stable, however VG not so much, so it's more likely to react badly and react with a plastic bottle if it is in one.
I'm a bit of traditionalist when it comes to the subject, as technically it's an aging process more than it is a steep. This is probably due to my experience with wine making and trying to understand what happens in that field. If you want to look at wine as an example that we know a bit about, most wines are force aged these days, however it is hidden in the processing as a pasteurization and filtration process to remove live yeast. Pasteurized wines undergo a forced aging, but this may remove any chance of the wine actually improving much more by aging.
In any case our e-liquids don't normally last that long, so it's not so much of a critical point. If it (heat in the car) works for one juice, fine, but it probably won't work the same way for all juices. Some may not be affected (hardly) at all, while others may be adversely affected.