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.
A series connection will not work if one bulb fuses but in a parallel connection if one bulb fuses the rest will stay lit.
wrong bulb in the rear wrong bulb in the rear
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.
Replace the rear bulb and be sure the contacts are clean.
If the current is stronger then the bulb will be brighter.
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?"
Bad connectors, and moisture. The caps must be on all ports on the headlight assembly. use connection grease on the bulb connection. replace with a new one from GM. other wise clean the terminals and replace with a new bulb. this works for a while but you will have to replace with a new connector socket.