Hi Tony,

Based on my research.

"In some cases electrical contacts on the removable bulb holders get dirty or partially corroded, and then don't carry current well. This can cause the Brake Lights or other Rear lights to fail completely or to fail intermittently. Clean the contacts in the bulb holders and also clean the contact areas on all the bulbs involved. Use a small piece of emery cloth, or a piece of steel wool. You can also check the connections where the wiring connects to the lights. Clean away any corrosion and make sure the contacts are tight. Sometimes you can use needle nose pliers to gently tighten them.

Another component that fails consistently is the bulb failure relay. When this occurs you will get a light on the dash to come on meaning the relay is faulty."


http://www.swedishautoparts.com/XC90/FAQs.html#ELECTRICAL_CATEGORY_-_Light_problems