99 Olds Bravada-exhaust check valve failed, melted my wiring harness. How do I fix it?

New wiring harnesses are REAL expensive and rarely purchased except when they're under warranty and you aren't paying the bill anyhow. Instead, trot on down to the local salvage yard for a used harness. If you have a pick-your-own-parts yard nearby, you can see where all the plugs are that you'll need to get to on your vehicle. This is one of the few instances when replacing the entire harness is preferable to repairing the few damaged wires.

To be safe, try to find a harness from the same year vehicle just in case there were some changes made. Look for the same engine size too, and the same options, (cruise control, air conditioning, etc).

I've had that same valve fail on other brands of cars with the same rubber hose melting so it's not just a GM thing.

That valve that failed for me was on a '78 Chrysler LeBaron. The only symptom was a loud thumping / roaring noise when I took the lid off the air filter housing. There weren't any wires near it. I plugged the hose off with an old Christmas tree light bulb! That stopped the hot exhaust gases from flowing up there so the hose survived many years like that. By the time I learned what that valve did, there was no point in replacing it. The car ran fine without it.

I'm not exactly a GM expert but I think the Blazer is the same vehicle. GM is really famous for having a severe lack of interchangeability between their various models, so the best I can suggest is to look at the connectors and wire colors to see if they're the same. There's usually a paper tag with a part number on it too that you can compare. If they're the same, they're the same. If the numbers are different, don't panic right away. Sometimes the harness could be identical but a little longer because it has to reach to a component that is located in a little different spot on one model. That is enough to warrant a different part number. On some models, there could one or entire wires missing. They do that to save money when a vehicle doesn't have some option. For example, there could be a dozen wires in one harness and five of them are for the cruise control. If a car doesn't have cruise control, they'll leave those five wires off but use the rest of the harness for the other things. That doesn't sound like much savings until you realize that could apply to a million cars. They just saved a few million feet of wire. The new replacement harness will have those five wires so they don't have to keep multiple versions in inventory. While they saved a dollar on each original harness, they will charge you ten dollars more for that new harness. Those five wires just won't do anything. That's just an example of why there could be different wires. If the harness you find in the salvage yard has wires were they were missing on your old one, don't worry about them as long as the connectors are the same. If all you can find are harnesses that don't have enough wires, get two or three, then you can pull the terminals out of the connectors on one and put them into the other one to make up the harness you need. I've done that but I doubt it's going to come to that on your vehicle.