On a mk2 it will be the later fusebox. I'll look up the wiring colours to the relay, but prompt me if I forget. I'm not sure about the boot, we would have to try it! It would mean a direct connection to the BCU (Body Control Unit).
One of the relays in the passenger fusebox will have two purple wires, one purple blue, and a thin purple black. You need to ground the purple black to sound the horn. So one of the spare outputs from the quasar: set as pulsed, connect one end of the relay to ground, the other to this purple black.