the horn is simple , a horn a relay and switch, and fuse 15?.
but is complex by other factors.
1: the clock spring . no air bags in 1991, so very easy.
there are 2 ways to diagnose any horn..
1: hot wire it, does it now honk? if yes, hot wire the relay does it honk now?
2: or use any voltmeter ever made and find out why.
#2 is the save way.
id hot wire the horn and make sure that 1991 ground bolt
mount is really still at ground potential. id bet that bolt has
long sense rusted, and is useless, like most are by now.
the fuse and relay are both in the PDC
the PDC means, engine bay fuse box in gringo. (non jargon)http://www.jeep4x4center.com/knowledge-base/index.htm?utm_source=cj#service
here is 93 for free.
see horn page one and the 2 tests.
or see the schematic.
at there, see the fuse box see fuse 15 (PDC 15?)
same chapter, see PDF page 90 (sheet33)
as you can see if the fuse is good.
12vdc arrives at the Horn relay on 2 pins.
if the horn switch works the relay ENERGIZES (closes)
and 12vdc arrives at the horn, and it honks
if while honking, the horn casing is 12vdc,
then means the horn is no longer grounded, a common happenstance.
if the relay dont energize check the horn ring contacts or connectors to it for continuity.
the relay can be tested, in the car.
pin 2 and 5 are 12v
pin 1 = 0 volts
pin 4 is 12v, if 0v the relay is bad.
easy huh., ?