Water can enter the taillight and even the rear hatch area of a 2005 Acura RSX if the gasket between the taillight and the vehicle body has been damaged or leaks due to age, allowing water to enter the housing. Repairing the taillights will require their removal and examination, along with replacement of the gaskets.
Use a hydraulic jack to lift the rear of your RSX and place it on jack stands. The factory jack points are in front of the rear tires.
Crawl underneath the RSX and look up to find the two Phillips screws that are underneath the taillights. You will need a long extension to reach them.
Open the rear hatch and pull back the carpet to expose the rear of the taillight assemblies. Use a 10 mm socket with your ratchet to remove the four bolts that secure the taillight assembly, then remove the assembly from the car, followed by the gasket.
Examine the taillight. If the taillight is not cracked and leaky, then examine the gasket. Chances are it will be chewed up and damaged. The new gasket will attach only one way, since the taillight bolts are spaced irregularly on the housing. The gasket will have sticky adhesive on it, particularly near the bolt holes. Slide a new gasket onto the taillight, then reinstall the taillight, making sure to reinstall the securing bolts along with the Phillips screw that secures the taillight from underneath.
Apply plastic sealant if you discover the taillight is cracked. There are a variety of clear silicone waterproof sealants, available at any hardware or automotive supply store. Clean the taillight with a degreaser, then spread a small bead of sealant on the crack, covering the crack completely. Let the sealant cure for a few hours before reinstallation.
