Didn’t really do anything TO my sprinter, but for use in the sprinter.
Made a passive solar water heater today.
I used this extremely hi tech system a few years back in my Westafaiias.
A plastic Sparkletts water bottle spray painted flat black. Laugh if you want, it's easy, almost free, environmentally friendly, and works as long as you have sun.
My typical use was to rinse off the salt water after a quick lunchtime surf session. I would put the 90% full 1 gal jug on the dash while parked facing the sun. It’d heat up fine. Take it down for the 10 min drive to the surf spot, back on the dash it went when I parked, windows up (carefull not to spill any water on your sprinter dash!). It’s Just enough for a quick warm water rinse. In winter I would fill it with hot water at home or work and wrap it with a towel or wetsuit for a bit of insulation. Anyway it worked then and will work now for quick rinses after surf, beach, mountain biking, hikes etc, or when camping in sunny locations. If heating it up outside choose a sunny location shielded from wind, and laying at an angle toward the sun helps too.
I always fill to about 90% to leave room for expansion on hot days. Note smaller or narrower bottles heat quicker due to less mass.