9 month old only naps in car
When my daughter was that age, I literally would plan my day around her napping in the car. I'd just plan adventures that took a bit of driving to get to, she'd sleep on the way there, we'd have a fun day doing something I enjoyed, she'd sleep on the way back. It was by no means ideal, but I got through it by just giving in and making it as indulgent a time for myself as possible.
If it makes you feel better, she did grow out of it, and then did a period of the best naps of her life - in her bed at home, which was shocking. I'm not quite sure how I made that transition work, but I think it was mostly her own maturity that made the difference. I think the phase was 9 - 10 months or so, with another bumpy patch when she transitioned to 1 nap at 14 months.
OH - I just remembered how I got her to sleep in the house. I jiggled her. I held her in my arms on the bed and jiggled her to sleep, kind of mimicking the movement of the car. That was also the same age that she would fall asleep to hard, rhythmic thumping on her back. I really had to meet her at her energy level and then ratchet her down. I swear, I should have won some sort of award for the intense effort I used to get that girl to nap.
I noticed that during the day, she only nurses to sleep when she is REALLY DEFINITVELY tired. For our baby, that just means she is slowing down, looking glassy eyed, and beginning to yawn, but before that DESPERATE FOR SLEEP time when they are impossible to calm down.
It goes contrary to a common piece of advice, which is that you should put them to bed much sooner than that, before they are super tired.
However, she does sleep better overall when put to bed less tired. It just means putting her to sleep ways other than nursing.