donnita keller

I have a 2006 Rx 330 Lexus and my tail lights for some reason the big ones will come on only if their are not on auto setting but the small ones won't...how can I tell if its a short or the bulbs out and sometimes I have to tap it on the drivers side only and the big one comes one but the small ones still doesn't.. I dont know what to do to figure out if its the bulb or a short and if its the bulb am I able to change it out with out buying the whole assembly part