Truck is making sparks when u start it up

I have a 1992 chevy 1500 i got a new radio and had it installed. Later on that night i was driving and then smoke came out from under the dash. Of course radio didnt work then but also the starter was dragging. So we replaced the starter. Check the battery (which is only 3 months old) to make sure no dead cell.  Everything ok , Hard to start if u could get it to . Then when i starter it up there were sparks coming from the wire thru the firewall for the thermostate guage.  We have taken the new radio out, disconnected the thermostate so no power going to it. replaced the started, and the battery and the thing still wont start  HELP