I can combine two of your false statements in one answer. First, the idea that radio does not work. Second, the one you raise here about only thinking about two brands.
First, I NEVER said "radio does not work". I did question how well it works and how one would measure results.
The ongoing campaign created awareness and established the dealer as wanting the business of Spanish speakers. So as those who were looking for that range of cars got closer to the buying point, the idea of visiting that dealer became top of mind until the time that they actually went car shopping. From that point on, it was up to the dealer to convert the ups to sales.
Being a Spanish speaker yourself have you stopped to think that the main attraction for the Spanish speaking car buyer may have been to consummate the deal in his or her native language was as important as other factors? Obviously the ads publicized the fact that the particular dealer operated in Spanish so that part of the ad worked. And given the number of native Spanish speakers in El A I would think the market was also good-sized. I would have almost expected that outcome.
As to your other point - that advertising works best when the potential customer is already considering a purchase - I agree, although it can work at other times as well. Three years ago I owned a Hyundai Santa Fe and I was perfectly happy with it and not looking to replace it - until I saw a story in Car & Driver about the Genesis. Went to a dealer and took a test drive and read some more feature articles (not commercials) on the vehicle and decided I wanted one. It was a very limited production car in those days so not many dealers had one. Also, the numerous Hyundai car ads most often did not include the Genesis (or Equus) because they were so scarce. So advertising played no part in convincing me to buy, timing me to buy or locating the place to buy. I will admit that the Genny purchase was not a commodity as would a Chevy be so it is different in that respect. This isn't meant to attack your story but rather to say other factors are frequently involved.
Most radio advertising is local. In most cases, it's the cash register that is the real "ratings" for a buy.
And I have also said that many times. You have to measure the business generated by specific ad campaigns to most accurately measure the effect of advertising - no matter what the venue.