I was wondering about your chihuahua-how sensitive and what size? I have two chihuahuas that are very different in personality and in the cold. One is smaller and will nosedive into a blanket even on a warm day-she is very susceptible to the cold while the other one won't tolerate a blanket around her and is even restless in the carrier. I have a winter coat for her that is extremely bulky for when we go up north; even during the long car trips in the winter,. She seems to do ok with that (this is very thick so she is even a little more restrained and rests more with the blanket). We are very careful, even checking her ears as the time can be more then six hours from PA to upstate NY and we stay travelling at night.
The other one works very well with a sling. She is very small (only reason I asked size) so she can cuddle in a blanket literally for hours and be extremely happy to the point of not wanting to come out. I can put her in the sling wrapped up easily. She has a coat but we alternate with that depending on the amount of response to the cold. Hope it helped-we have indicators that are easy to detect and I have a bag always prepared with more blankets. We also use the pet stroller for any trips; even from the car and back to the vet. The one we have keeps their fuzzy blankets in piles and they both usually dig when they start to feel more even when we aren't checking. We check ears and shaking constantly but honestly we haven't had any problems with more then needing to add a blanket or hug them to us a few moments. They love the stroller; both of them.
Only to add, I tried the petcarriers that are like backpacks or front packs and for these two, at least, it didn't work out so well. Neither liked them nor were so comfortable to not try to move around or sigh in frustration. Even walks should be fine after trying your little one out in it. Just always recommend to be prepared and have back up. It can always be cut short then.
Enjoy your [hopefully] white Christmas!