92' Rear Right Speaker Works About Half the Time

Thanks for the help on removing the door trim. However, I have encountered another problem. I got all my new speakers installed, and they all worked perfectly for about a month. Now, however, I have noticed that the rear right speaker (above the back right tire) only works about half the time. I will be listening to the radio, and for no apparent reason it will just stop producing sound. After about 20 minutes, it will come to life again and play like nothing happened. I checked the hook-ups in the trunk, and there is nothing loose or disconnected. There is also no crackling while it is functioning so i dont think the speaker is broken. Any ideas what might be causing this and how I might go about fixing this issue?