that doesn't sound all that simple but I'll take your word for it.what horn is it? what kind of controls does it have? if it just has a normal momentary switch, you can get away with a small nice looking button style switch in the cab. if it is a musical horn, then you might have to have a control board mounted to control the different tones. either way, it is always possible to do another switch.
for example, and a regular aftermarket horn. mount the horn and the tank. ground it. take the switch for it and run it to the cab. you will end up running 12v to the switch. take the other end of the switch, run it to the 30 pin on the relay. run 12v to 86, ground 85. run a wire from the 87 pin to the + on the horn. forget about 87a. simple as that.