I use hot glue for holding down wires. I have two guns, a low temp craft gun and a standard high temp gun. The glue from the low temp gun sets fast, the high temp has more working time but can melt thin plastics. I’ll usually place a large drop where I want the wires to go, let it cool for just a few seconds, then shove the wires in the pool and hold them a few more seconds until set.
Experiment outside the car with some wire scraps on some scrap plastic and I think you will decide that hot glue is a good choice.
EDIT to add: The glue for low temp craft gun has a lower melting point. This is a good thing for working with it and not burning fingers, but direct sunlight on a dark surface might be enough to melt it. The high temp glue is much more difficult to melt after it has set.