The fuse you used is fuse #66, and it's listed as "Run Accessory Feed - If Equipped". Here's the wiring diagram for that circuit:
View attachment 7074
View attachment 7073
So yes, it's an unused circuit - perfect for tapping in to.
Just an FYI - the common terminal of the relay, #30, is fed by a pair of 80A fuses from the PDC. The relay is engaged in ACC/RUN modes by the green/gray wire energizing the coil.
Hopefully, you tapped into the load side of the fuse, not the line side - otherwise, you'll have up to 160A of unfused current at your detector, in the event of a short