tail lights

i have a 1989 buick century 4 door auto 3.3liter 223000 miles. My tail lights went out but everything else works brakes turn signals etc. I replaced the fuse and it popped when i turned on the light switch. I assumed it was a short at first and checked the rear lights they were all blown. I disconnected the rear light panel in the trunk and put in a new fuse it still popped so i tried to trace the wires under the trim board it is very good looking with a solid plastic sheath on it. I am assuming now it is not a short. could something be pumping to many amps into the fuse. As a side note I recently replaced the radio to a cd player and had to put the wires for the dash light on it like it was on the original radio unfortunately it is now on all the time even before I turn on the headlights. I am fairly certain the tail lights were working for about a month after i replaced the radio so I am unsure if this could have anything to do with it