2002 Malibu, tail light issues

I have a 2002 Malibu and the tail lights are blowing the fuse. Also, when that happens, it seems that the inside speedometer panel lights also go out. It is sometimes hard to see the panel at night and it is illegal or cause for a ticket if I drive without the tail lights.

We checked for shorts and none were found (this is from the GM mechanics).

I have had the back lights go out before, they fixed the electrical system. That was years ago. Now the tail lights keep going out. I will keep fuses around for the time being.

What do you think if a short isn't the problem?

It has been raining lately (winter season), I was thinking that water gets into the tail light system and shorts them out. But wouldn't the bulbs also go out?

Also, if it is water seeping into the fixture, how can I better protect against more leakage?