I've been leaving my tackle bag in my truck a lot lately. Have any of y'all done that and the sun melted your soft plastics?

NO.

I leave soft plastics in the trunk of my car year round and I'm in South Florida, they are in absolute pristine condition.  I do not remove them from the package until I use one.  I don't use senkos too often, have had a pack in my car for a couple of years.

in AZ i never do.

Nope never have. *knock on wood*

It has happend to me once, but they didnt melt they just got softer.

They have never melted for me.

If you are using quality plastics, then a car will rarely ever heat to the point that it will melt them. Will it cause colors to bleed, sometimes.

Yep, before I started leaving plastics in their original bags, I had many a bleed from bait to bait.  Haven't had any melt though.