I’m going against the grain to say it doesn’t matter. It just doesn’t. If this is a symptom of other things going and there are ACTUAL problems with disrespectful behaviour and discipline, deal with them. But who sits in the front? Man, it sounds like you are both kids. If you’ve slotted into their lives and this is the only problem, it’s a small one. Pick your battles and be grateful. Is this actually a hill you want to die on?
Many children feel sick in the back of a car. Car sickness is weird, and she may not have realised the back was making her feel odd. My DD sits upfront because she throws up in the back. It’s not an issue because it’s just us, but if we have a lift anywhere, she rides shotgun. It’s not about respect. I don’t need the front seat to show that I am a powerful adult in charge of everything that goes on in the family. I know that. She knows that. It’s no less ‘respectful’ for her to have the front seat. It’s just meeting everyone’s well-being with the resources we have. If your DD needs the front seat to feel good, let her. It’s no big deal. If you, as an adult, NEED the front seat to confirm your family status, I think that says more about you and your adult insecurities than her or relationships and her immature ones. If this is around transitions to her mum’s that’s going to be a tough time for a kid. It’s stressful for them to change houses, and she maybe wants her dad’s support, or just her last little bit of time close to him. Is she getting any time just with her dad? That’s important, especially if she has gone from having a lot to none. She can like and be adjusting well to you being part of her family AND still want some one-to-one, and find the transitions tough. The control in particular sounds very anxiety-based. She’s telling you she’s finding the transitions hard. You can set a boundary, allow space for her anxiety and her unique and special relationship with her father, because you are an adult. She possibly can’t, because she isn’t.
If she’s speaking to you poorly, not respecting your possessions, time or body, those are issues. If she just wants some one-to-one with her dad, that’s a valid need.
Stop going on the transitions at all. Let her have that special time with her dad. Extend it to include a bit of a treat, if possible. Of course you sit in the front when she isn’t there - it’s silly not to, just as you don’t expect her to sit in the back if you are not there, and don’t avoid that. If you must go, or for other trips, toss a coin or let her choose. Remove the power, remove this ridiculous idea that gives (either of) you status, stop feeding her anxiety, but give space for her need for one on one time with her dad, and acknowledge that transitions between homes are hard for kids. I bet it will disappear in a few weeks if you can rise above it and meet her underlying need.
If you insist that you must sit in the front, because it gives you the respect due as an adult, you are feeding the troll. She feels powerless and disrespected and anxious on transitions to mum’s and sees the front seat as a way to stop that. So by insisting on having it, you are reinforcing the idea that that’s how you get it, and that’s what you need. And she will push harder to get her needs met. It will be one very desirable, the way it has become to you from your childhood. If you acknowledge it’s no big deal, look at the family as a whole and how everyone’s needs can be met, and what’s really causing the anxiety, she will learn a lot about collaborative problem solving and looking for root causes, setting firm and fair boundaries AND trying to meet everyone’s needs with the resources you have. Which is more adult? There is no benefit in trying to teach children they can’t always have it their way, by insisting we always have it our way.
I would also use it a dry run. Family meeting etc, etc. You will encounter actual problems as she gets older, so use this non-problem as a great way to establish a protocol for dealing with them.