1989 GMC s15 - radiator fan constantly running

[Zane M]

If it is a stock 1989 S15 fan set up, then it will run whenever the engine is running because it is a mechanical fan attached to the waterpump.

If you have some aftermarket/custom electric fan set up, then it could be the temperature switch that controls the fans is not functioning properly.

Now where that switch is on your truck? I don't know, as it is not a stock set up. Start following wires from your electric fan, and you will find it somewhere.