The front speakers are the same for the Blazer and pick-ups. Top of the dash in the center (oval) and top of the dash, far right and far left (round). The left and right use a special bracket so if you get them from a junker make sure the bracket is with them. There is no factory provision for door or kick panel speakers.
The Blazer (civy) also used speakers in the rear corners by the tailgate, again, special brackets. The pick-ups (civy) used speakers in the rear corner of the cab, once again, special brackets.
No need to drill anything, just remove the plastic filler plate from the dash bezel. There is no connector on the wiring harness for a radio.
When salvaging from a junker, take everything! Radio, speakers, knobs, brackets, nuts, screws, wires, plugs/connectors.
Good luck!!!