"The Mighty Ten Ninety" was first used for the Tijuana area 1090 frequency shortly after Wolfman Jack bought all their air time for five years (1966-71). In those days, it was XERB. At the end of those five years, the call letters were changed to XEPRS because Wolfman owned the U.S. trademark for XERB, and was not about to give it up to the owners who decided they could run it just as well as the Wolfman had.
Wolfman's slogan was actually not original to the market. The Mighty Six Ninety was applied to the Tijuana station on that frequency in the late 1950s when it was English language Top 40 and run out of San Diego. Before it became All News XETRA.
As the logo above attests to, the slogan is still being used today for XEPRS, now an English language All Sports station serving San Diego.
As for the Wolfman, despite his death nearly 20 years ago, he is still remembered. A couple of days ago, two thirty-something construction guys were doing warranty work on my house, and I overheard them talking about Wolfman Jack. They knew him from American Graffiti, but loved the music show on TV that he was part of, which I presume meant they were watching re-runs of The Midnight Special, since they weren't born until after the original series left the air.