My DD2 has what is still an undiagnosed condition, but has many traits of autism among her other issues. We get the complete meltdowns for unexplainable reasons too and going in the car when she doesn't want to (for some reason I don't understand that she can't convey) has been a big issue for us too.

There are times when I've been able to put off the trip, but there are times when we simply can't and just have to get on with it. Like you, I feel absolutely terrible physically forcing my little girl into her car seat but sadly there's more than just her in this family and as much as it breaks my heart, she can't always dictate what we all do. Also, if we're 40kms from home there's not much choice but to get in the car!

As the others have said, I always try to explain to her what is going on and why, but there are days when she is not calm or rational enough to understand the explanation. Routine helps, but you can't always do things by routine. The most effective way we've found is to give her tons of warning of what is going on. So, sweetie in an hour we need to go to X. Then its "don't forget we need to go to X in about 30 minutes". Then 10 minute, 5 minute, 2 minute and 1 minute warnings. It doesn't always do the trick, but a lot of the time it is reinforced in her head for long enough that she's quite accepting of it. Same thing used to go for the medication she needs to take each morning - it used to be an enormous fight but we did the countdown thing and now she will actually come and tell me if I've forgotten to give it to her.