GM-GMC: wiring a stereo, k5 blazer, dash lights

Question
how do i wire my new stereo to a 1988 k5 blazer

Answer
Get yourself a test light. You will need to find a few essential wires.  You will need to find a constant power source (B+)  It should have power at all times regardless of the position of the key.  The second wire is another power wire which only has power with the key in the on or acc position.  The third is a ground.  After you have found the B+ wire, clip your test light to it and check the solid black wires for a good ground.  You may have a fourth wire depending on the original stereo.  You may have an illumination wire in your factory harness, which will dim the lights the same as your dash lights.  Ground your testlight and probe other wires with the key in the on position and your headlights on.  Any wire that only has power with the headlamps on is your illumination wire.  Connect all those to the mating wires on the new stereo along with all the speaker wires which you should be able to figure out.  If you don't have an illumination wire on the truck and your new stereo has one, hook it to the ignition feed wire (the one with power only when the key is on)  It will turn the stereo backlighting on when ever the key is on.
Hope this helps.