From GM Bulletin:
Lamp Condensation
A distinction is made between a lamp with condensation and a lamp with a water leak. Condensation appears in a lamp as very small droplets of water, a fine mist, or a white fog on the inside of the lens. It occurs when the air inside the lens reaches the “dew point.” This is the temperature at which the humidity in the air is cooled enough to become liquid. The most prevalent time of the year for fogging to occur is in the spring and fall.
Most exterior lamps on GM vehicles are designed to expel accumulated vapor through a vent system. Although the vent system operates at all times, it is most effective when the lamp is on and the vehicle is moving.
The amount of time required for the lamp to clear may vary from two to six hours. Customers with short commutes will experience a longer time to clear the lamp.
If these conditions are noted, advise the customer that replacing a lamp assembly may not correct this condition.
Typically, the condensation is located primarily in the lens corners (near the vents) and does not cover more than half of the lens surface. (Fig. 5) The condition should clear of moisture when the vehicle is parked in a dry environment, or when the vehicle is driven with the lights on.
Fig 5
Water Leak
A water leak is evidenced by numerous drops of water in various sizes, collecting on the inside surface of the lamp lens after the vehicle has been exposed to rain or a car wash. (Fig. 6)
A water leak condition will cover more than half the surface of the lamp lens. In addition, there may be an accumulation of water in the bottom of the lamp assembly. This condition will not clear when the vehicle is parked in a dry environment, or when the vehicle is driven with the lights on.
Fig 6
Lamp Service Tips
When diagnosing or repairing a lamp for condensation or a leak:
• Do not replace lamps for fogging.
• Do not modify the lamp housing, such as drilling holes.
• Do not use the incorrect bulbs for the application, which may damage the housing and connector.
• Cold water in a car wash on a hot day may cause headlamp fogging.
• Use care when removing and installing lenses to prevent damage to the housing and mounting tabs.
For example, on the 2014-2015 Silverado 1500 and Sierra 1500 trucks, when removing and installing the tail lamps, do not apply excessive pressure to the lamp lens using your thumbs. Use your palms to pull or push the lamp. Applying too much pressure on the lens can cause the housing to crack behind the reflector field of the lamp.
When servicing any exterior lighting assembly, be sure to transfer electrical connection seals to the new part. They are often on the body side of the harness connection, but may have stayed on the part being replaced. If not transferred to the new part, the connection may corrode due to water entry.
Use of Non-GM Lamps
Many aftermarket companies manufacture lamp assemblies that look very similar to the original equipment (OE). Non-OE lamps may be installed on vehicles as part of a collision repair.
If these lamps are holding moisture or the bulbs are inoperative due to corroded electrical connections, they are not covered by the New Vehicle Warranty. Check for the GM trademark on the part label. (Fig. 7) Not all lamp assemblies currently have the trademark, but future assemblies will contain the trademark on the label.
Parts without the GM trademark are not eligible for replacement under warranty.
Fig 7