Yes, it is disgusting to find that in the glove compartment, but I've also found half-eaten McDonalds in the center console before. Others have reported finding porn in their trunk.
While it is a bit "disrespectful" to not get a call back, he did call you back once so you are basically playing phone tag. In the course or normal business this happens all the time. Yes, it would be nice to have him be a bit more proactive, but for all you know he is dealing with a PC member whose car died on I-35 or perhaps a general member who crashed their minivan. While your problem is a drag, you can throw it out and move on. Other issues might take longer to resolve. If I were you, throw it out, enjoy your trip, and write Hertz when you get back (or talk to the manager at the return) -- you can probably get a discount or $ off the rental.