I removed both housings and changed one of the dead bulbs.
Poured out whatever water I could and since it was still raining, I just 'spooged' 'em sparingly and judiciously where I thought it would do the most good, including a thin film around the bulb housings.
I even went as far as putting a bead around the top corner and across the top to help redirect water to drip straight down under the car. If I have to remove that, it won't be difficult.
I'll have to wait for sunny days for the water condensation in the taillight housings to evaporate, though.
They're a double wall design with an outer lens and the inner reflector lenses. I couldn't justify trying to disassemble them and end up maybe breaking something.
When I turned the housings upside down initially, water poured and dripped out of a crack at the top of the lenses. That'* where I put a thin bead of silicon.
Also, on trial fitting for reinstallation, I noted that there was a possibility of the water getting in around the top of the gasket between the housing and the body, so, I spooged it, too.
We'll see how my handiwork turns out one of these days when we get some sunny weather and the first rain or carwash after that!