I often hear one of these expressions when the other one is required.

Problem:
These two expressions are not synonyms.

Explanation:
My wife recently heard a man say, “I will wait on you in the car.”

The preposition “on” bothered her, and we discussed how frequently each of us has heard “wait on” when “wait for” was required.

The common English blunder seems to be to use a form of “wait on” when a form of “wait for” is required.

Someone who “waits on” someone else is acting as a waiter or waitress.

So “I will wait on you in the car.” literally means “I will act as your waiter (or waitress) in the car.”

The man whom my wife heard should have said “I will wait for you in the car.” because that person was not saying that he would act as a waiter in “the car”.

Solution:
Use “… wait on you …” when you are a waiter or waitress; otherwise use “… wait for you …”.