This business about warping in sunlight and in cars is OLD information. Material chemists have made huge advances in the last couple of years.
The modern pla just doesn't seem to do this.
I've tried the hot water sink thing - just feels a little slimy - doesn't bend or buckle.

And no car gets hot enough to melt plastic. Some of the pla I have doesn't even extrude till it hits 200C, it's hardly likely to melt in a car.

Really guys, you need to try and keep up with the way materials are going. And the pla I'm using just doesn't behave the way everyone keeps telling me it should.

The exact same part made with abs and pla will crack with abs and not with pla. Again this is the opposite of the old information of how the two plastics stack up.

Even the cheapest pla I've got is less brittle than the abs.
I can jam a slightly larger pla part into a hole that a slightly smaller abs part won't go into without cracking.
Both printed with same infill and shell settings.

My biggest problem is that I now havea shelf full of abs that I just don't need or have any desire to use.