What could cause water retained in spare tire area of trunk?

You have a leaking seal somewhere. Check around where the spare tire sits. Is there a rubber seal that has a rip or tear in it or maybe a kink? That's a good place to start.

Usually it is the trunk lid itself- does this happen after it rains? My guess is, if the seal around the lid looks good, then you can adjust the trunk latch to make the trunk door fit tighter

This is common for Lucernes. Mine leaks too. I've done a lot of research and have found that the leak is due to two holes, originally patched by GM with low grade tape, which are located below the break lights. The bad news is that you'll have to take the rear bumper off to locate the holes and reseal them.

Took my '07 Lucerne to Kudick in Mauston, WI. They claim never heard of such a problem, wanted minimum $400 to take apart car and research. Found leak coming in around tail/break lights, like above. My question, will GM make the repair as warranty/poor design item?

Will Go Make this repair on 2007 Lucerne