There should be a thread entitled "Things you can get at home depot for your car" . I bought a ton of stuff for my cars, clamps, screws, materials, nuts and bolts, wire, fluids, cleaning products, mech gloves, tools etc.. The best part is the prices are like $0.46 for screws and clamps where ford wants $8.99-$20/ea .
The reason for the 3 or 5 speakers is I have a ford explorer and only one side of the rear cargo panel can be used. The other side has the gas filler wall , so 2 in the doors, 2 in the back seat panels, and 1 in the passgr side rear cargo area , or 2 less if the amp can't handle 5 speakers.
I bought 12g wire for $1.45 CAD per meter. About 50 cents a foot and it worked great. I did a dry run in the back with a test box and I had no problems. I am doing a simple improvement over the factory base strereo , nothing special but it must not have a box .