Jimmy's Riviera

My friend has a 1998 Buick Riviera that he bought used and loves. It has 120,000 miles on it. It’s white …

But, had a leak that no one can find. He finds water under the back seat and in the trunk. It is NOT rain water. He leaves the trunk open from time to time to help dry it out cause it smell musty. Can you please help him with some clues ? Thanks