You have a leak!
Duh, you knew that though. I'd check the rubber gasket that is the trunk seal. Any cracks, breaks or missing sections will let water into the trunk. If that looks OK, I'd check the rubber seal that lives between the entire tail light assembly and the body of the car. Same deal. If it's bad, replacement is called for. Commercial car washes use pretty high pressure rinses. That might be why the water is getting in there.
My 2002 is a 2.2L and the Water Pump is on the (standing in front of the car looking aft-towards the back) left hand side. Locate the Serpinetine Belt and it's right underneath and just aft of the alternator pulley.
If the 2002 Pontiac Sunfire works at times and the fuse is causing trouble. There may be a short in the fuse box or electrical system.
It turns the back up LIGHTS on, on the rear of the truck.
On my 98 sunfire, the tail lights and marker lights only work if you turn on the headlights. If only the daytime lights are on, the Pontiac on the trunk is illuminated but no other lights at the rear until brakes or turn signals are activated. This is common with a lot of vehicles out there. Daytime running lights are not necessarily the same candle power as the headlights. I can tell a big difference in mine.
There are a few reasons that would cause your 2002 Pontiac back speakers to only work sometimes. The most common cause is a loose or corroded ground wire.
hire a dectective
check for bare wires or short in the wiring to the rear lights
Assuming you mean brake lights, the brake light switch is broken or out of adjustment.
You might be able to get one through a dealership, but I'll bet they're danged expensive. I'd check with local junkyards.
I need help on fixing my back brakes on my 1997 sunfire
I have a 2002 Sedona - after opening the drivers door turn your side lights back on you can then close and lock your car with the side lights on. Hope this helps.
If the lights work a little but appear dim then i would suggest a dodgy earth?