Weird. I've never had that problem, except in cars that leak or have standing water inside for some reason. Then I have the problem bigtime, so severely that no workaround will work.
For instance last winter I suddenly got rust blooms. Digging into the spare-tire well, I found 10mm of water in the bottom. Mucked it out, problem never returned. (and I don't have A/C, so A/C dehumidify is not available to me.) Turns out in a visit to the snowbelt, I had hauled some stuff that had been laying on the ground and was encrusted in snow and ice, and some melted.
So the big question to me is...
Where's the water coming from?
One-time spill, as in my case. Assures 100% relative humidity, causing repeated problems until addressed.
Water leaks. Even a tiny leak can do a lot, even if it hasn't rained - it is letting condensation on the outside of the car inside.
Wet clothes, often. If it rains or snows a lot, your shoes and clothing can bring it in, especially if you take it off and leave it in the truck.
Exhaust leaks. Engine emission controls these days are asymptotic to perfect, so there isn't much carbon monoxide etc. left to kill you. However there's an enormous amount of H2O (water), with exhaust over-saturated with water. As it cools, it must condense.
Faulty air valves bringing in saturated air from outside. Cars should leak air just enough that people sleeping in a car don't get CO2 poisoning. At full highway speed, the venting is designed so aerodynamic forces force a small amount of air through your ventilation system, enough to push out a possible exhaust leak. Of course the blower needs to work on full. However the car shouldn't be drafty. One compromise is done with passive, one-way air flapper valves in certain places. You might check the health of those valves, if equipped.
I honestly think you must have one of the above problems, and the answer is fix it.
However if you are at wit's end, it may be worth looking at a dehumidifier. A desiccant dehumidifier can be small and work well at low temperatures and pull relative humidity down to near zero. I don't know whether this can be sized to an auxiliary battery fed by solar. It takes a lot of power for its heater, so you may need to plug in.