I asked the same question regarding the wiring harness and this is what I found out. There is a harness that goes with a 90's era Mazda (no model specific) that fits the speaker outputs on the stock Alpine amp. The Mazda system only has 4 wires, but you can put another 4 wires to make all 4 speakers work (by adding pin connected wires). As for the power/ground wires, you can use a standard flat connector to plug into the stock harness (it's only a 2 flat connector plug).
As for the head unit, I found enough wires with pin connectors to make a connector to fit into the car's wiring harness. The guy at the shop told me to divide the pins with thin cardboard, plug them into the car's harness, and then use a hot glue gun to lock the pins in place. The glue is not an electrical conductor, so it won't cause a short in the wires. My idea to keep glue out of the car's harness was to put a thin sheet of plastic in between the pins and the car's harness until the glue dried (like saran wrap or something simliar, the key being something that won't melt). Once done, you can plug the wires in and use a plastic zip tie to keep it in place.
I don't believe you'll be able to use the stock head unit since it runs it's signal through wires vice an RCA cable (I may be wrong, so please chime in if you know a way to convert the signals).
I know this probably sounded confusing, so I'll try and take some pics of the harness I had done and what I plan to do to make a custom one. Overall, this setup will have run me between $25 and $30 for the harness and all the extra materials.
Good luck and I'll see what I can do to post some pics.
Frank