two things:
#1 its a company car you are leasing
#2 its a low budget starter in car computer
to me, I interpret this as that its maybe a play toy, an experiment, something to use as a learning experience, etc. furthermore, that its NOT your car, that, as you said, even replacing the radio i a big deal. how about running power lines through your firewall? that amplifier isnt "installess" either!
the fact is, an FM modulator is your best option. its a lease and not your car. Its temporary! why spend the money on proprietary adapters that are worthless to you after you have to give back the car? its an experiment (im still making this assumption) so why drop big money just to experiemnt with other parts of in car computing?
furthermore, I assume you get most of your MP3's from internet sources, yes? what kind of quality music do you think you are getting? about the same as FM radio anyways!
There is only one argument I can think of that would have any weight here agasint the FM modulator. If you live in an area where the ENTIRE FM dial is filled with stations, I can see the modulator presenting a huge problem trying to compete with some signals.
as long as you avoid modulators such as models made by BOSS and pyramid, stick with modulators built with more reputable components, such as those available form
www.davidnavone.com , then in fact you can get a very nice sound. competition quality? nope! but does that really matter when the speakers in your car cost 87 cents anyways? remember, bottleneck!
I recommend FM modulation. not transmission, something you actually have to pull out the radio and physically plug in to.
when you get the hang of in car computing, have a more solid idea in which direction you want to go, know your advantages and disadvantages, have power supply issues ironed out (how many members here say FM modulation SQ sucks then turn around and make a post complaining about awful alternator whine?) AND have your own car you want to start doing some more permanent modification to, THEN I would have no reason to suggest a $20 box could yield the same satisfactory results as spending $200 on a new radio!
but for now?
in the car you are borrowing?
with your budget?
and goals I predict you have?
FM modulation is nearly the only way to go.
good luck!