Yes, they have to establish a "foundation". In the most basic terms, it means they have to offer evidence and a reasonable explanation (relevance) of why they believe Caylee was in the trunk.
For example, a perfect one would be the hair with the death band (if allowed). Canine alerts should do it, too. There are probably several others that aren't jumping out at me at the moment.
I really don't think the prosecutors wil have any problem establishing a foundation to get the stain evidence admitted. The defense likes minimizing it as a "minor setback". okay...whatever brush they want to paint it with is fine by me.
ETA: I asked on the Verified Atty. thread more specifically what would be required to lay the foundation...
AZ answered me here.