You need to be more specific on what kind of car. Most commonly a bulb holder gets burnt contacts and cleaning with steels wool or scraping with a sharp object will help but is usually just a temporay fix. Common mistakes are improper bulbs or installed backwards. Alot of cars have a poor ground wire issue.
Need to check the ground and the connection were the bulb goes into the socket.
Open the hood. look at the rear of the headlight housing. Rotate the bulb holder counterclockwise and pull the bulb out. Disconnect the plug from the bulb. Wipe the new bulb with a clean cloth if it is dirty, or you get your fingerprints on it, or it might burn out.
bad ground or severed connection
That's a serial connection.
wrong bulb in the rear wrong bulb in the rear
To fix a rear headlight that’s out on your 1999 Jeep Grand Cherokee, first, check the bulb by removing the rear light assembly. If the bulb is burnt out, replace it with a new one of the same type. Ensure the connections are clean and secure before reassembling the light assembly. If the bulb is fine, inspect the fuse and wiring for any damage or corrosion.
Replace the rear bulb and be sure the contacts are clean.
Check the following...........turn signal bulbs front and rear, and brake lights, probably a burned out bulb or a bad connection due to corrosion in the bulb socket, finally check the turn signal flasher, under the dashboard, if the bulbs turn out to be OK. To control corrosion in bulb sockets use a di-electric grease that seals the socket to bulb connection and repells water.
where can I find a video on how to replace a 2006 rear SIDE MARKER bulb>
How do you replace the rear brake light bulb on a 2001 Honda civic?"
remove old bulb, insert new bulb.
Lightbulbs flicker when there is a problem with the electrical connection or the bulb itself. This can be caused by a loose connection, a faulty bulb, or fluctuations in the electrical supply.