Just picked up a '03 S2000, prepping to upgrade the system (Alpine CDA-105, older Rockford Fosgate Amp). I'm going with Modifry's solution to keep controls, and also tucking a Sirius module away to give me Howard Stern (SCVDOC-1 and KCA-SC100 working together to get my Stratus 6 connected). I'm coming from Miata's with Bose (awful) that had to be worked around wiring-wise (bosectomy), and involved running completely new wires into the doors. I'd rather not have to be taking door plugs apart again if I can help it.

If I'm mounting the sub in the trunk, will I just take it's speaker wires and head back to where the OEM head unit connected to the speaker wires and reconnect to/re-use that wiring, or do I need to run new wires to get to my new speakers in the doors?

 

Lets clear up what your target goal is. You mentioned a Sub, but also door speakers. Are you questioning the install of a sub AND door speakers on a stock head unit? Ive replaced the headunit, added a sub and door speakers, each with their own amp. Im here to help

 

It doesn't have to be a nightmare to run speaker wires into the doors.
That said, if you aren't putting crazy power through them, you can connect the speaker wires from your
headunit or amp to the adapter harness and use the OEM wiring into the doors. There are no crazy
impedance issues as with Bose.
Speaker of adapter harnesses, buy yours from Modifry with the DCI - he sells two versions - one that you
splice yourself to the headunit harness, or a plug&play with all the work done. Many generic Honda harnesses
(including the one above, unless things have changed) don't have the s2000-specific dash & mute wires.

 

Goal is to replace head unit and door speakers, while also adding a subwoofer in the trunk, and an amp to run it all. Not a big thumping system, just a little stronger, better sounding.

In light of that, I'm considering bringing the post-amp speaker wires out of the trunk, back to the area where the speaker wires to the door mounting locations already are, and connecting the new system to that, rather than running new wires to and through the doors. Given I'm not running anything crazy power-wise, I'm hoping I can re-use that stretch of wiring to avoid having to open up the wiring that goes through the door hinge area (Did that in a Miata, not fun)

 

I have picked up the PnP harness from Modifry, awaiting delivery.