HI, my daughter is 10 mos. and her brother will be 3 in a mo.
This probably isn't the answer you want to hear, but why not build the car naps into her schedule.
thanx for advice - the morn nap in car works ok b/c she naps for about 45 min. but afternoon nap is always cut short and then she is exhausted but wont nap at home.. she adores her bro (although he isnt aways nice to her) and the sec. she sees him she refuses to nap - she senses when he is near.. so being in room wouldnt work - my son also woke up at the 9 mo. age but after a week or so of CIO he resumes sleeping well most of the time -
3y.o. aren't always nice to anyone.
she does almost always go straight back to sleep.. who knows maybe she does need food? idk