4 lo switch

[LS1GMCTruck]

It either was part of your original wires coming from your old tranny, and/or it came out of the passenger side firewall group of wires. You need to get your year trucks wiring schematics. They are invaluable for this problem & for many other questions you will have in the future. Contrary to popular belief, they are very easy to read & understand. I bought mine on E-Bay, in my year trucks GM Service Manuel. Don't buy any aftermarket Manuels, buy only the Factory GM Manuels. They usually come in a two volume set. One covers mechanical & electrical(which you need) & the other volume covers useless stuff like doiing bodywork, etc. You can buy just the one Manuel of the set. Lots of good used Manuels out there, that are still in very good condition.