Location of horn relay for 1990 Chevy k-2500 5.7l

The fuel pump relay should be under the hood on the firewall, square with a 3 in it. Hope you can read it?

I think the amplifier is under the RH side of the steering column and the horn relay is attached to it, but i believe your problem may be a ground issue. I'd keep mapping that diagram you have and keep looking. https://www.f150forum.com/attachments/f10/85895d1329757315-location-cruise-control-amplifier-horn-relay-screenshot070.jpg https://www.f150forum.com/attachments/f10/85894d1329757302-location-cruise-control-amplifier-horn-relay-screenshot069.jpg

In the fuse box under the hood or in a box behind glove box.

I need the model year?
It is possible your owners manual may give location of horn fuse and relay, depending on your model year.
The horn itself is usually at the front of vehicle.

I found this info for horn relay.
If more than one relay, hopefully you can see wiring colors at relay, then look at horn relay wiring diagram and match the wire colors to the
relay.
I'd check applicable fuses first. Any electrical circuit needs voltage and ground.

horn button only grounds it,
put 12 volts to the horn it self see if it works,
if it does then check for horn relay

The relay is located under the glove compartment. Little difficult to get to but if you peek your head just underneath, you'll see it. Easy to take out and replace. Ive replaced mine and still have a problem. I hear a clicking noise and even can feel the contact when horn is pressed. I changed the horn about 1yr ago and now am wondering if the horn could have gone out so soon.

under the dash in far left top corner iis a small panel inside are the flashers relays and buzzers which are difficult to remove and install.

It could very possibly be that your horn relay has gone bad. I believe you might find it on the inside of the front fenders or the inside of the radiator support. I'm an ol' Chevy guy so don't quote me!