1970s 76 eldorado stereo & cig lighter help

[clamparillo]

Hi,
I just got a 76 eldorado and wanted to put a new retro fit stereo in. I carefully took out the dash and disconnected the radio. The blk / yellow wire that is supposed to be the (only) power is not showing anything on voltage reader. The pink wire for antenna was already disconnected..Also the cig lighter which was working , is not now along with a couple of dash bulbs after I was testing live wires and wire had a small spark. Also found a random disconnect black wire.. not sure what that is.  All the fuses are good.. any help would be greatly appreciated.