The biggest issue is connectivity of the hardware.
Here's three scenarios
that exist within my own group of friends/family:
Car #1 - Stock car AM/FM radio with cassette player
, with 10-disc changer in trunk. Tape player does not work with any "converter" tape due to how it was made. It plays tapes, and only tapes. The CD player is from the 1990s and only plays Red Book CD audio on CD-ROM or blue AZO CD-R. (Phthalocyanine "silver colored" discs won't working without severe skipping. Green cyanine discs fail to work at all.) No MP3. The best that can be done here is to get a radio transmitter that will convert stereo mini output (headphones jack or line out) from an MP3 player, and then tune the car radio to that channel. It can work pretty decently. There are several iPod FM transmitters on Amazon
. These were designed for this exact purpose. I used one myself, back in the early 1990s, before in-car CD players were affordable, but a discman + FM transmitter was only about $100 (vs $300+ for the in-car stereo, either as dash board deck or trunk disc loader/player). The FM transmitters run about $25 for good ones. Lesser price for cheap ones, which can be hit-or-miss in quality. They also make transmitters like this for cell phones. Very handy for long road trips! If the car could have used a cassette adapter, then this would have been my suggestion: http://www.amazon.com/gp/product/B00...SIN=B0007R4LI8
Car #2 - Stock car AM/FM radio with CD player
, one disc at a time. No changer, nothing fancy. Just a radio with a CD, no tape deck in this newer model truck. No MP3. Only plays standard Red Book CD-Audio on CD-ROM or CD-R. It also works best with AZO discs, and can skip a bit on other types of CD-R.
The solution here would also be the transmitter. Again: http://www.amazon.com/gp/redirect.ht...reative=390957
Car #3 - Owner-added car stereo with MP3 option.
This was originally installed as a car CD player, but was futureproofed with wiring that allowed it to later be wired with a USB extension that would play MP3. All music must be loaded to a thumb drive, with no obvious folder structure limits (beyond 3 tested), and the radio is used to select the song. Not the safest setup, in my opinion, trying to scroll songs while driving. Best used for "load and go" use -- the owner makes me nervous diddling on the radio while driving. Yikes! These days, you can grab a stereo like this for under $75, plus installation.
For example: http://www.amazon.com/gp/redirect.ht...reative=390957
. I would buy online, and have a local place install it. Don't let a friend/buddy do it, don't buy/install from Best Buy (overpriced + incompetent, as far as I'm concerned). A place like Car Toys can install it pretty low-cost.
.... and that's really the only option. Those three scenarios cover most of what you'll see out there.