One reason why you Chihuahua may not like being in the car is because of motion sickness. Many people get this and so can dogs. For some dogs, it can be an issue of feeling confined and this can cause quite a bit of restlessness.
There are a few things to help with this:
1) Use the right canine car seat. While one option for pet owners is a restraint belt, this does not work well with the Chihuahua breed. You'll want to obtain a raised car seat that brings the Chihuahua up high enough to be able to look out of the windows.
Motion sickness occurs when the body feels movement but the eyes to not see it; this can cause health issues
such as nausea, dizziness and for some dogs, vomiting.
2) Timing of Food -
If a dog tends to vomit while being driven, a meal before leaving is not a good idea. If you must take your Chihuahua in the car, try to time it to drive at least one hour after eating so that food has a chance to settle.
About 20 minutes before leaving, feed your Chi a light snack. Crackers work well, as they can help to absorb stomach acids that may churn up.
Once you have your Chihuahua safely in his car seat, offer one of his favorite toys; a small Kong filled with peanut butter works well as a distraction.
4) Window -
Open the window a bit if the weather allows for it. Even though your puppy
or dog will not be able to stick his head out of the window like some larger breeds can do, feeling the fresh air rushing in can help a Chi feel much better and have a better sense that his body is correct that there is movement.