Its really not.

Pick up a general soldering iron and solder at your auto parts store or electronic components shop. Sand the area of the tail lamp that makes contact with the bulb holder. Then apply solder to the contact area on the tail lamp. Sand that area down as much as needed to come close to flush with the original contact metal.

OR

If your instance isn't severe enough where you need to add solder to provide a contact for the bulb holder, you can follow instructions provided by NC X5 Guy.

OR

Do it the easy and less complicated way...buy a new bulb holder/tail lamp (Whichever is the culprit in your situation.)