So a couple months ago I started having issues with tail lights, then for a while one headlight on low beam was out, then it came back, then the reverse lights - then the sunroof was intermittent, then the rear wiper stopped

Jumped it once after a dead battery - fine, but still with quirks

Two weeks later, after cold spell - completely dead battery, tried to jump - nothing

Got new battery - while installing, noticed water in the well next to the battery/under the driver taillights/fusebox

Installing new battery, got spark when I connected neg as last step. But then locks made sound, put key in, got the ding, turned key, two clicks come from back of car and dead again

What's up? Is it water getting in? A short? The line going to the front? Fuses/relays? Have a good mechanic but want to be informed before going in. They do all kinds of cars so not sure how well versed in Volvo electrical they are