Firstly, just to verify whether it is the alarm sounder/siren (mounted on the bulkhead just to the right of the expansion tank) that is sounding continuously, or the horn?
The horn sounding continuously is almost always due to a failed i/c in the Pektron SCU, and it is usually necessary to disconnect the battery or remove the horn fuse to shut it up. Obviously, you then need to remove the SCU from behind the air con evaporator at the back of the dash centre console, with is a bit of a challenge in itself! The SCU can then be repaired (contact Paul Brown at https://www.technozen-electronics.co.uk/
). For more information on the Pektron SCU, I would recommend a read of this thread over on mg-rover.org: https://forums.mg-rover.org/how-do-i...cu-bcu-384938/
If it is the alarm siren, then it is likely that either the sounder unit has become water logged and needs to be dried out or replaced (the security shear bolts holding the guard casing over it can usually be removed with careful use of Mole grips); or there is a short in the wiring between the body and the tailgate - the wiring frequently gets chafed where it passes through the rubber boot at top right of the tailgate opening. Pull the boot back and thoroughly check these wires and repair if needed.
The alarm sounder has its own back-up battery within the sounder unit, and once triggered will continue to sound and will not normally stop if the main car battery is disconnected.