that's the only place unless you fancy searching for a under-car route... but that'll mean removing the underpanels and potentially drilling the car for p-clips or something to secure the wiring. my install fully concealed in the sub-boot area.
picture time since i'm pretty useless at describing things:
this is for the 9g typeR so engine bay could be slightly different...
lower than you think in the engine bay:
theres two thicknesses to the rubber grommet; thick in the center and thinner closer the edge. I poked a hole through the thinner bit with a small screwdriver and then poked the power wire through with a bit of WD40 to aid pulling it through
on the inside, you can get your hand up the back of the heater box but don't need to remove the heater system. I assume you found it from the interior side, but if not, remove the panel on the passenger side and slide your hand up the back of the heater box. it's not far... 5-6" from the bottom edge of the heater box.
the rest is fairly trivial, but i'd recommend removing passenger seat & rear seats. maybe 30-45mins work. so much more room to work.
made a false floor to the boot as well to mount the audio it. not bolted to the car and just an interference fit:
this was the original install but has since changed a lot. removed the various quick connections as they caused issues but kept the power distro blocks. also upgraded to the Audison APBX 10 slim sub with fits perfectly on the right side of the amps for a fully concealed install. vastly better than my ancient vibe blackair evo box taking up the boot.