Man...great questions...very similar to ones I had when I was bumping my auto modeling up to the "next level"
I like putting nos in the trunks of my cars. If you can't spring for a kit in resin or aluminum...I've occasionally used the end of pens. You'd have to do a little searching for find ones with the right curve/size. Then, building the details with a little styrene.
For speaker boxes, I've cut the shape out of little blocks of wood. Then, I bought some wire mesh at michael's. I use (again) those little metal rings that separate the two halves of a ballpoint pen to put a round edge on the speaker.
Carpet the trunk...I use paper tape from the first aid aisle. Then just paint over it whatever color you want. Cheaper than buying fancy flocking.
SO, on to engine wiring. I found the most efficent thing for me was buying an engine detailing kit from PSF hobbies. I bought it off ebay, but you can order it from them directly. It come with almost everything you could want...multiple colors of wire, braided lines, heater hoses, seatbelt material. Its about 17 bucks, but it totally paid off...you can use it for more than one model.
Also, if you get a hold of some books specifically based on modeling car details, I've found those very helpful for ideas. Try your library...or just glance through them at your hobby shop.
Good luck...I'm sure your neighbor will love whatever you do!
-jonathan