So, what's the magical answer?
Unfortunately, there is none. That said, there is one golden rule you can keep in mind when heading out, so your baby is comfortable and you can stay sane.
Dress your baby similar to yourself. Really, it's as simple as that.
As new parents, we have a tendency to "over dress" our little ones, without even realizing it. Then, as we're cruising down the highway, the baby starts crying in the backseat, and we can't figure out why. Many times, they're just over-heated.
If you're wearing a long-sleeve shirt and a light weight jacket, your baby should be fine in the same. If it makes you feel more comfortable, bring along a light weight blanket to throw over the car seat or stroller, just in case.
When it's snowing, be sure to put on a hat and mittens; the same holds true if it's just really windy and cold. Keeping their ears protected from the wind is just good sense.
Avoid swaddling them in multiple blankets - even when it's really cold outside. You have to remember that, they're usually not outside for very long, and so anything you put on them to protect them from the elements is going to overheat them while they're in the car.
Remember, when dressing your baby for cool weather, take a look at yourself. Although you don't have to dress alike, following the same general rule of thumb will keep everybody just the right temperature.