As a first attempt, without looking at schematics, I'd use a relay. Find the two wires from the finger trigger, splice in a wire on each and run to the "input signal" terminals of the relay (84 and 85, I think). Run the 12 volt power into the relay power terminal (I think 86). Then connect terminal 35 to the positive lead on the horn. If you have a fuze-block like FZ-1, just use the output of FZ-1 as the power input to the relay.
This may be a llittle confusing, but should work.