"Wasn't Rush Limbaugh's show a local program on WABC-770 for a year or two before it went national??"
Yes, it was. For a year or two, Limbaugh did 10 AM-Noon on WABC, then did his national show 12-3, which WABC did NOT carry. At that time, Lynn Samuels was on 12-2, and Bob Grant started at 2. If I remember correctly, John Mainielli was the PD then and didn't believe a national show would go over in New York. So, to justify Limbaugh using WABC's studios, Rush had to do a local show. Eventually, of course, JM was convinced and WABC started carrying Limbaugh 12-3 and tried several replacements 9-12 -- there was Joy Behar for a while, Prager and Ed Koch, and probably a few others I've forgotten.
And to address what Qwerty mentioned about Grant and Hannity, he is right again. There's no doubt WOR's image as the home of advice and financial shows hurt Grant when he moved there in 1996 after being fired by ABC/Disney. Still, for the first couple of years, Grant still won the timeslot. Hannity did eventually win it, but right up to the end, Grant's numbers were strong and only slightly behind Hannity -- even with a weaker WOR line-up around him. But while on WABC, Grant rolled over WOR in his timeslot. And it wasn't as though WOR conceded the slot. Grant had two good competitors -- Gene Burns and Jay Severin (he was a bit calmer then than in his recent Boston days). Grant buried them both. In fact, as far away as this market -- NEPA -- Grant was usually coming in around number 12, 12-plus, and even higher in older demos. No one talk show host coming from just one station has ever had the dominance Bob Grant did at his peak.