Spit up is one of the hardest things to get out!
I have found the when spit up is still wet you can use baby wipes to clean it up. So even though it's old and dried, try some baby wipes.
Thanks Joy - I will try them!
Like Joy said, I have found that baby wipes are the best for that! I didn't want to use anything harsh, especially on carseat straps. Always worked for me!