Please route a separate wire from the battery to the amp. Make it the correct (large) size to handle the load of the amp. This is not cheap by the foot, but you won't be running it very far if the amp is in the trunk (boot). I think I used 4 gauge, which is pretty fat and a large fuse holder close to the battery for safety. The better quality wire has a lot of very fine wires inside which will flow much more current and is easier to route out of the way and into tight spots. Amps running subs need large amounts of current to move quickly. Use bigger wire to the sub from the amp too. Allow air circulation to keep it from overheating.
Any aftermarket deck will have a blue wire out the back to turn on the electric antenna any time the deck is turned on. Some have two remote turn on leads, one for the power antenna and one for amps. Google will tell you a lot about proper wiring of amps before you buy, but the challenge with a Z Roadster is the small cabin forcing you to get creative with the enclosure.
By the way, mine is a sealed enclosure, not vented. Google the difference. Basically, sealed equals tighter; vented equals more volume, but boomier and less precise. It is tougher to get a custom box with a port dialed in to sound right. I wanted the music to reproduce accurately and deeply, not to impress the car passengers around me with excess thumping.