Finally solved a massive headache of a problem. The Grand Am's are notorious for swimming pools in the front floors. Eventually, I was pulling out gallons. After inspecting every square inch of the windshield and door seals, I was baffled. Popped the hood, stuck my head under the driver's side of the dash, and there it was. The surrounding for the bundle of wiring that comes through the upper corner of the firewall was completely rusted, leaving the wire loom free to move around. And water to flow down the windshield, through the cowl vents, and into my carpet. Compressed air and Great Stuff took care of it, thankfully!
Now, on to the brakes. The fronts are terribly noisy, almost an embarrassing amount of noise. I don't have the tools or space to do it, but I have a shop that said $20 a wheel if I'm bringing in the pads (or shoes, for the rear drums). I'm hoping any brake drag from the front end will be fixed, and coasting should improve. It's teaching me the valuable art of engine braking though, and I've nearly mastered it to avoid odd staring
Currently at 280 miles, about 1/4 tank left, and UG is saying 33.2MPG for the urban commute.