electrical help

[goonie017]

im having issues with my 240fastbacks electronics ,mainly in dash,you see when i bought the car the previous owner had an after market stereo installed but took it out b4 sale , my issue is that da dash lights ,interior lights even the door chime dont work , when i replace the 10 amp fuse verything works great up until i connect the stereo,when i connect the ground wire of the stereo it sparks blowing the fuse and all interior lights go dead replace the fuse and it works ,until the stereo is grounded ,im guessing there a short somewhere jus dunno where to start looking ,ive known the 24o is notorious for stereo problems so im told but is there a way it can be fixed i mean i really want a stereo in ma ride nd where beter to look for help than other 240 owners any help would be appreciated thx