Step 1: buy cheap microfiber towels.
Step 2: wax car.
Step 3: remove wax from car with cheap microfiber towel.
Step 4: treat microfiber towel as a disposable item and use it to scrub down all the nasty bits under the hood, trunk, gas tank lid, and in the doors.
Step 5: throw microfiber towel away.
I used to use shop towels to do this, but now I’ve seen the light. $0.50 well spent.