My DS is almost 18 months and has just started having a day nap again.

He has completly gone off milk so no bottle to go to sleep but I find laying next to him for 5 minutes always works. I think he is going through a groth spurt he seems to be sleeping alot lately and hes eating alot more, so he may be over it in a couple of weeks and then hes back to just napping in the car again.

HTH