I have an 88 f250 351w that i think has a bad horn relay. My question is where is the horn relay on this truck?

 

On these models I believe your horn relay is under the hood, just behind the air intake box on the left inner fender.

dn.

 

A horn relay is used only on vehicles equipped with speed control. If you do not have speed control then you do not have one.
If you have speed control the the relay is mounted on the outboard attaching screw of the speed control amplifier module in the cab under the dash near the steering column.