Foam in a can will hold wires in place. Sometimes I use a hot melt glue gun to hold wires in place until the foam hardens. I also sometimes use an old garden hose to run wires through. Black irrigation tubing is also inexpensive. Like you said solid wires in a vehicle that vibrates and flexes is not a good idea. Solid wire bent enough times will brake. I also stay away from using black wire for negative. About the only time I see black used for negative is at the battery. Manufacturers often use black for positive wires in the chassis and white for the negative, (similar to house wiring). If you look at 12 volt light fixtures they come with a black and a white wire, (the white is negative, the black positive). Using red and black to feed them makes it a bit confusing to look at because you will have to attach the red to the black and the black to the white, (LED lights can be polarity sensitive so if you switch the wires around often the light will not work).
Thats a really great idea with black irrigation tubing, I wish we had thought of that. As of right now, most of the 12v wiring is foamed in place and we ended up replacing some after realizing we messed up with the location of an "ON" switch. We will definitely take your advice and use expending foam to stabilize the solid household wires and reduce their flexing in corners and such. The black and red 12v wire shouldn't be a problem since we both work with 12v systems pretty often (specifically, LED lights).
I also go with heavier wire than necessary. Not because I need the added amperage capacity, but because the smaller the wire is, the more resistance it has to current flow. Resistance causes heat. I am not talking excessive heat that is noticeable, but you are wasting energy creating that heat. The voltage also drops because of this resistance. This is bad news if trying to charge a remote battery. Point two volts, (0.2), can make a big difference if a battery gets fully charged. Often jumper cables are cheaper to buy than the wire itself. I use a lot of 4 gauge that comes in 20 foot lengths and just cut off the clamps. If I buy 4 gauge wire it costs about twice as much.
The jumper cable trick is something I definitely have to remember. I saw how expensive thicker gauge wire can be so it’s good to have a little trick to saving on it.
