I have taken the steering wheel apart and the 2 leads and wire are fine, all fuses in box under dash and hood are fine, pulled the horn and tested it on a 12v battery and it worked and now it must be the horn relay. Everyone tells me it is under the drivers side kick panel and all there is is a green square starter relay, a round silver tail light relay and the fuse box with no open slots for a 3 pronged horn relay like napa advertises. All I have to go on is the 93' service manual and it says it should be just to the right of the fuse box but there is nothing. I am stumped.

 

10 year old thread and I'm in the same boat. No idea where the horn relay is located. 10amp HORN Fuse under hood good, can find a relay FOR SALE on eBay for 1991 Toyota pickup but no idea where it is located on the truck. Also as OP stated, nothing at fuse box at driver side kick panel

 

Fuse/Relay block against driver's side kick panel just below the dash. This is for a 1993 Yota but I worked on a 90 and 91 and they are similar.

 

Does this white thing look like a horn relay. Never noticed it before, it’s cobbled on to fuse panel. No horn relay on fuse panel as pictured in diagrams

 

So,,had a pro take a look. There is no relay on 1991 model, regardless of what the manual say. Its a simple TWO wire horn. I learned the horn on driver side never worked. Someone had installed it and I traced the wires to where they stopped, not connected. During this I found the OE horn on passenger side which had corroded terminals. Had to cut wire back a couple inches as wire corroded initially poor ground. Replaced with a TWO contact horn from '12 caravan and all works.
While I was at it I decided time to KILL the warning buzzer. Removed circuit from back of fuse panel and cut trace contact with knife. No more buzzers. There is a thread on YOTATECH showing the simple procedure