I leave my fishing bag in my Tracker. It gets hot as dagnabit in there. Sometimes I leave a bag or two of soft plastics on the floorboard, but most of the time, they are in my bag, and thus, out of direct sunlight.

 

So how do plastics hold up to 120º+ heat throughout summer? Is it of no effect, or should I bring the bag inside?

Sometimes I’ll leave them in the car without any issues. Only thing is with some, especially swimbaits, they can get warped if they are bent. 
 

Anything Elaztech cannot handle the heat whatsoever. Those are the only baits that I make sure to bring inside

Good to know about elaztech baits. I haven't used any yet.

I leave my stuff in my truck no problem. Never really had an issue. I mean if ya threw them on the dash they’d melt but on the floor has always been fine for me in Florida and Iowa 

I left a package of Rage craws in my boat last summer and the clamshell warped but the craws were fine. First time that’s ever happened. It must’ve been dang hot under there.

I leave a lot of replenishment gear in the truck and haven't had a problem left. It's all in bins though, so no direct light on the bags.

X2 on the Elaztech stuff melting. Can't say I've had an issue with traditional plastics. 

Elastomer and plastisol baits can handle heat without melting.  They do become more pliable and take a slight bend or kink if they arent stored straight.  You can bring them inside or turn the AC on in the car and take a drive and when the plastic cools down the bends can set.  Plastisol is any easy fix, but elastomer is much harder to straighten as hot water can warp the shape and or melt the elastomer.  Elastomer baits do fine inside my jeep during the summer, but you must keep them in the bag and out of direct sunlight.  Elastomer can react with certain plano/plastic storage boxes.