That electric fan relay (if it’s the one I’m thinking of) is a stand alone unit that’s tucked away and almost hidden up inside the body metal under the air intake and behind the headlight. Not easy to access at all, you may have to remove the wheel, wheel liner etc to get to it. It’s large, about 4" long and approx 1-1/2" sq and dark green in colour.
From a rather fuzzy memory of the fans running for long-ish periods like yours, I believe there is a temperature switch, something involved with the A/C system(?) that turns the fans on and off.
My fans sometimes run for quite a while after shutdown but that’s usually in summer if the A/C has been blasting.
Check for debris/leaves in the condenser/rad area?
Oh Yeah, that relay looks pretty sealed up BTW …no opening this one up IMO
Thanks Larry, explains why I couldn’t find it, I made a mistake in the last post, the Haynes manual does show it to be exactly where you say ie on the left side (looking forward sat in the car) in the area near the air filter, however it doesn’t explain you can’t see or access it without removing a load of other stuff. My A/C hasn’t worked for years, but I do keep the condenser and rad area quite clean, usually even on the few scorching hot days we get here the fans will only run for a minute or two at most when I turn the engine off regardless of whether I’ve just come off a motorway or been stuck in slow moving traffic. I’ve had time to think since my last post, the thing is, although they seemed to run for longer today they did eventually stop, so it would be unlikely it was that relay that triggered the horns to blare, especially as it happened in the early hours of the morning after the car had sat unused for a few days.
Casso, the operation of your fans, as I believe you may have now concluded, sounds normal. The operation of the fans is dependent upon the radiator thermal switch that is in the lower left of the radiator, engine bay side. As shown in the wiring schematic:
there are three wires to the switch, a black (B) that provides the ground (earth), and two thermally-switched wires, a white (W) and a white/blue (WU). As shown, the switch for the white wire closes when coolant temp rises to 186° F, and will open when the temp falls to 173° F. This white wire goes to the fan relay control module where a couple of relays perform an interesting feat as shown here:
It gets a bit convoluted but the bottom line is the left fan motor is provided voltage via a purple/white (PW) wire and the right fan motor is provided a path to ground via a black/white (BW) wire.
To test the operation of the thermal switch in situ you would test for continuity to ground at the terminal (#2) to which the white wire connects. The kicker is determining the coolant temp. It might be easier to test the switch if it is removed from the radiator and placed in a pot of hot water, but that approach presents other challenges (draining the coolant).
Thanks Mike. Unless it REALLY starts acting up I’m quite happy to leave the fan operation alone for the moment, even if there is a fault, which I now doubt, because of the time period (3 days) the car was left unused before the horns suddenly started blaring, I can’t see the fan relay or wiring for that system being the cause. I’m praying the rain keeps away again tomorrow and I can move the car away from the house and try re-connecting the horns, and praying even harder that when I do all is quiet. I’ll keep you informed of how it goes. Cheers.
Casso, as previously discussed the horns must have 12 volts via the yellow wire to sound. To put your mind at ease (or not!) fire up your mutimeter on DC voltage and connect the + (red) lead to the terminal fed by the yellow wire and the - (black) lead to a good ground. No voltage, no blaring horn!
Doh ! I’ve been so stressed or anxious trying to sort this out I’ve overlooked the ‘simple’ tests I can do.
It’s rained again all day today so I sat in the car and put the metal plate with all the various control boxes back in place, the glovebox and bolster are also in and fastened up. Tomorrow rain permitting I’ll test those pesky horns with my multimeter before pushing the connectors back on to the terminals. Fingers, toes and anything else I can cross it will show zero volts 
Thanks Mike.
Today I tested the voltage to the horns with my multimeter and hurrah there was nothing, I got the Mrs to press each horn button and only then did the meter show 12 V, so despite the fact I’m still not certain what triggered the problem in the first place it looks like all my dismantling and drying has resolved the problem.
While I was poking about under the bonnet I noticed a flaky bit of paint on the bulkhead beneath the plastic box that houses the wiper motor. The thick bundles of cable that run beneath it were hiding it, the thickest bunch has a piece of sponge like material wrapped around it, I’m guessing to prevent chaffing against the bulkhead. This sponge was sodden wet and where it rested against the metal a few tiny rust pinholes has started, this is most likely the entry point for the rainwater that wet the sound deadening on the bulkhead inside the car and caused it to become soaked. I’ve started to remove the wiper motor and box to access it so I can repair it properly tomorrow. I’ll also scrap that piece of sponge and use some rubber strips I have in it’s place. Fortunately the remainder of the bulkhead is sound, when I first bought the car 12 years ago I painted the whole area with clear waxoyl having learned from my old Sovereign that it’s a prime spot for rust to break out and that has preserved it like new. Once I’ve treated the pinholes I’ll wash it all off with brake/ clutch cleaner and then re-apply a fresh coat of clear waxoyl to the entire bulkhead.
The musical duo Simon and Garfunkel celebrated it in song: The Sound of Silence. Congrats!
when I bought my 93, I had the same problem and I figured out it was due to the battery being weak and it was a built-in security feature that can’t be turned off from what I could figure out. mine was set to the off position and even pulling the fuses to both the horn and security system (different horn for the alarm which is right behind the headlight and in front of the air box) I put a oversized battery in mine and that worked until the battery got drained due to a power draw from the fog lights not turning off (bad switch in my case, almost caught the car on fire and one of the bolbs melted through the plastic airdam ) I just removed the siren/alarm from the car and haven’t had a issue since.