Cmayna,
The hot lead grounding itself against the socket would cause a dead short and blow the taillight fuse.
The bulb does ground itself thru the socket. Power enters the bulb thru the base, goes to filamint and exits out the brass section of bulb. The brass section is touching the socket itself, which is grounded. IF you have 12 volts in base of socket w/o bulb, you either have a bad bulb or ground.
Try taking a test light to the terminal in the bottom of the socket and grounding it in differnt places, including the socket or light housing. If the test lite works with a different grounc, try running a new ground.