yes you can still use the battery terminal on your piggy back solenoid.
Just copied this out of an FAQ I did on another site. should help
This guide will work for both twin tone air horns and musical, main difference you will need to find room for all the horns on a musical set..
1st find a good place to mount the horns. I chose under the n/s wheel arch because the slats in the inner wing made an ideal place to clamp the horns to...I made a bracket to mount the horns to first, then clamped that bracket to the slats. Make sure if you mount them here they do not foul on the wheel, check wheels at full lock both ways
Next the compressor unit, if you use the tubing that comes with the kit you will have a limit how far from the horns you can mount the compressor. I was lucky there was already a hole in my bulkhead sealed with a gromet that I was able to utilise for the mounting of the compressor
Next wiring find a suitable place for the relay, again no servo on my mini so I have a nice open space to mount it
On the compressor you will find two terminals postive and negative, the positive you run a cable from that to a terminal marked 87 on the relay.
The negative I connected to the same bolt that mounts the compressor to the bulkhead.
Next on the relay terminal 30 you connect to a permant live feed, the live side of the starter solenoid is a good source for this, I've been a bit rough here in that I ran out of ring connectors so the cable is clamped by the two ring terminals already on the solenoid, I will replace this once I buy some ring connectors. You will need to fit a 30 amp inline fuse on this cable.
Terminal 86 on the relay goes to earth, again Ive used the car body, the black wire attached to the mounting screw of the relay
Finally Terminal 85 goes to the horn switch, easiest way of doing this is to unplug the spade connector that plugs into the existing horn and plug your cable you have attached to terminal 85 into the wire that went to the horn.
