Split 12V, 12V ACC, ground and illumination wires from the stock harness to a new male connector and the HU connector.
Dimmer wire should go directly to the male connector, although I think the only difference that makes is that if you have the dimmer on max, the display will no longer dim with headlights? Maybe also does the reverse? Hardly useful either way.
Also a MICU cable. No idea if it is used or not, but probably safer to wire that to the male connector to.

So, female connector to male/new HU connector. Male back into stock radio. If you have a Honda->ISO then it could be tricky. Best to buy a female to bare wires, male to bare wires and use the stereo's bare wire connector. Use solder and heat shrink tube to join. Note there are no SWC connectors on the metra harnesses though. Can order 2 female harnesses and swap over pins or if you are using pre-outs, reuse the speaker pins.

HVAC controls should work as usual after that.