Water in trunk under the spare tire ??

drill a hole in the bottom of your trunk. its a cheep fix or you can get new weather stripping for your trunk. ether way its just rain water

check the seal around where your trunk lid rests on. mat be a crack or you may have a small hole under the spare tire compartment. If you have carpet then you may need to cut a small square out of it where there is more watered pooled