I recommend you contact the brand company & they will tell you the closest dealer/service authorized to make repairs. Even if you’re skilled it’s good to get the repair dealership recommendations.
Sure, You can find cheap plug in heaters, butane or propane heaters, under the dash mount heaters from Amazon. You could even go full DIY & get a diesel heater mounted in the bed with dryer vent hoses everywhere.
If you want it done right & actually keep your dad warm, a good dealer should recommend a heater that’s tied into the cooling system & is not under but inside the dash. Yes, just like in your truck. Just my opinion but do it right or don’t do it at all & just use blankets & hand warmers.