Why do you put the wine in your trunk? Do you ride around with no AC? If you have AC in the car, why don't you put the wines in with you?
The fact that the temps in the wine cooler can fluctuate isn't that big a concern. The air in the fridge can and will have much larger swings than that wine in the bottle. You take a bottle out of the fridge and put it on the counter and it still feels cool for quite a while. So your wine isn't getting cool and warm and cool and warm even though the surrounding air might be.
In any event, what you're describing does not sound all that much like your wine was cooked. Maybe the Mondavi Napa Cab just wasn't as good as you remember? There are better wines in the Mondavi line.