As for the 'combo' type antennas, they were never the 'best' thing going, always had complications. If it's just for 'looks' then make it look nice but don't use it. If you want to use an antenna, I'd replace and seal one of those roof mounted ones.
(Just for grins, I used to work out of one of those @#$ things. Compared to other types of ambulances, they were the pits. No room to speak of, always 'hunched' over. Had to learn to 'duck walk'.)