I used 12/2 landscape wire for a pair of 12v/55watt work lights on a roof rack. No issue with the 10 amp load and the PVC jacket was rated -20*C to +105C.
If it gets hotter than 105*C [221*F] on the roof of that truck I think problems with landscape wire will be the least of my issues.
[Landscape wire is easy to find by the foot in 14 ga and 16 ga. - Ace and Lowe's both carry it. I had to go to a landscape supply to find 12 ga. by the foot.]
It is probably overkill, but for the price I don't know that I've found anything I like better. It's the 14ga stranded copper wire from Home Depot. My biggest complaint is that they don't sell enough colors. Anyway, it's got an oil & gas protective coating on the insulation. It's a little bit of a hassle to un-coil it especially deep into the spool, but it works well on non-insulated crimps which I have finally made myself start doing.