Not to gross you out but my son puked in the car. I scrubbed like mad and got rid of all remnants but the smell. I tried everything under the sun without success. A neighbor suggested I crumple up newspapers and toss them in the car. It worked like a dream. She said newspaper soaks up odor...
Try vaccuming it really well with an attachment. Once you have as much odor out as possible, they also sell mattress protectors which are like a big plastic bag that engulfs the mattress and zips at one end. That might keep the smell from coming out too. It would be good to use also as this is a used mattress...
Hope this helps