My back Tail Light doesn't go on.

I will have to take your word for it that they are both in the same bulb. That used to be very common but not as much any more. When bulbs used to be responsible for both the running/tail/parking light and the brake/turn/flasher light they were specially made with 2 filaments in them. One filament was for the brighter light (brake/turn/flasher) and the other filament was dimmer (for the running/tail/parking light). It is absolutely possible that only the dimmer filament for the running/tail/parking light burned out. If you pull the bulb out and look at it you can probably see both filaments and note that one is noticeably thicker than the other (that is the bright one). Sometimes when the filament burns out it is obviously broken loose and rolling around inside the bulb. Sometimes it still looks good and even works sometimes because there is only a small break that sometimes still makes contact. There will be a number on the bulb so you can get a replacement at any autopartsplace or even wallyworld. Maybe a 3057 if memory serves me. Shoud cost less than 5 bucks. But could simple be a bad contact or wire and the bulb is fine. You would need an electrical tester to check that out.