How do you treat old car plastic parts to prevent them from becoming brittle due to heat?

Pray? Only advice i have..

When you clean the engine bay spray tire foam on everything. It will attract dust but it will keep plastic in good shape even when hot.

KY Jelly.. It not only makes you look like a mac when you buy it weekly..
But it will actually keep all plastics in good shape and doesn’t attract dirt.. nor is it flammable..

Replace the parts for new ones.
If you can’t find them, get them 3d printed.
If you can’t find that then spray lots of wd-40 on them before even thinking about moveing them.
Plastic is like a death sentence. You are just waiting for it to break.
If you can have a metal alternative then use it at all times..

Also when removeing plastic parts, it’s a smart move to shake them a little before applying force to loosen the gunk that is working as glue.. For example, a plastic nut will have higher chance of not breaking if you first lightly tap the socket/wrench to kind of “vibrate the material” before applying your oomphh to it..

Do not ever overtighten plastic. I fell to this mistake once and it costed me to remove whole instrument cluster just to get access to the offender cause it’s unremoveable now. All because it was a left hand thread and not a right hand thread.