1976 Dodge Van Sportsman 360 Engine

The left rear tail light went out so I changed the bulb and soon thereafter I have been blowing 20amp turn signal fuses again and again. The left rear signal light does not work..The left rear brake light does not work. For a short time with a new fuse the right and left blinkers work but flash really fast. I hit the brakes and then the fuse blows. The turn signal lights on the dash are very dim and un-noticable.. Where should I check for a short. ??? Bad engine ground to body?? Poor light sockets due to moisture and dirt and rust? Flasher?? Turn signal switch?? Wrong bulbs used for some reason..?? I'm at my wits end.. Would it be a poor fuse block or ground to the fuse block??

Please advise..Thanks Stoshee