99 TL: front passenger side carpet is wet

[ker]

Just started noticing wet carpet on the front passenger side of my 99 TL. I don't think it's coolant - just water. I think the 2 things to check are
(1) heater box drain may be clogged or loose? Is there such a drain and, if so, where do I check it?
(2) moonroof drain tube clogged? Could that cause this? If so, where do I look for that?
(3) is there another possibility I'm missing?

Anybody have any other ideas on the source or what is best approach to trouble-shoot?
Thanks.