Yes as Richard says its an independent circuit and always use diode protected relays in all positions!
There technically is an MOT issue with most Morgans as most fall outside the cut off date allowing air horns to be installed. First thing to check is have you just got one horn fitted behind the grill/stone guard. If so it is probably the low tone klaxon. The car is wired for two and the wires are just taped up, the mounting hole is also there. Just buy the high tone option or a kit with both in and fit the high tone as well. You will be very pleasantly surprised by the difference in both sound and volume and nothing else will need to be modified to fit this addition.
That's what I did, original was useless.
