1992 Mazda B2200 tail lights

Electrical problem
1992 Mazda b2200 4 cyl Two Wheel Drive Manual

I recently changed the factory radio to an aftermarket radio. After the installation was done I noticed that the dash lights and the tail lights were not working. The fuse was blown and when I replaced it it blowed again. I got a volt meter and checked continuity. There is 12v on the primary, but when I turn the swith on the secondary goes to ground. I removed all the lights and still no change. I unhooked the new radio and still no change. Any ideas?