Look up the Head-Fi/Audio Glossary of terms, but for a quickie reference:
i. DAC - digital to analogue converter
ii. Amp - amplifies an input signal to drive a transducer/driver(s), which can be whatever is in a headphone, IEM, or speaker (technically speaking, "speaker" refers to the transducers themselves and whatever they're mounted on, usually referred to as cabinets, or however they're installed into a car)
iii. DAC-Amp - in headphone systems, this refers to a device that has both a decoder (DAC) and amplifier in the same box. Circuit design varies - types of digital inputs, where each section gets its power (some DAC sections are powered by the host device, ie, a computer while the Amp runs off the power supply that is in it), etc. Speaker system equivalents are usually referred to as receivers, beginning with amplifiers in the 1980s that had a DAC circuit built into it.
If you want to enjoy MP3's on a car, you don't need a DAC-Amp designed for a headphone system, at least depending on what car you have/what receiver is in your car. ANY receiver that uses any digital format - be it the "ancient" ones with CD, or the newer ones that also have USB inputs for HDDs or iPods and Androids (some Alpines don't even have optical drives anymore) - technically has a DAC to decode the digital audio file, and technically also some kind of amplifier to drive the car's speakers. The amp - or DAC-Amp - used with headphone systems are not the same circuits used to drive speakers.
What car do you have anyways, and what receiver is in it/what inputs does it have? Most newer cars have a USB input, and assuming your car does have this, even if you are looking into a headphone-oriented DAC to have better sound than your car's receiver, improvements may not be as discernible as when using such with lossless files (as you say, MP3's is most important); plus, in a car environment, sound quality heavily depends on transducer placement - you cab use the best DAC and amp available, but if you don't custom install the transducers and use time alignment tweaking tools, you will still hear the driver's side tweeter first, followed by the driver's side midwoofer, then the passenger side tweeter, then the passenger side midwoofer, then the sub. Delays are in microseconds so they are not always obvious, but take for instance how the vocals are either nowhere discernible or straight ahead of you while one of the guitars is still far off to the opposite side of the cabin (in a real recording, vocals are always center; in a concert, the performer can move around the stage, but the speakers broadcasting his/her voice don't move around with him/her). No amount of high-quality decoding will solve this problem, and if you hear sharp treble (like cymbals, or sssss and TTT!!! in the lyrics), it's because you have soundwaves from the high frequency transduccer bouncing off your windshield and you hear the closer one before the other.
Edited by ProtegeManiac - 11/10/13 at 7:36am
Also, consider the convenience and ergonomics - if you hook up the analog output of a DAC to the 3.5mm analog input on your car's receiver, everytime you change tracks you have to look down at that device or hang it (and not as stable as the dash on its own) somewhere, whereas some kind of storage device plugged into your factory receiver's USB input means you can see and navigate the contents as you should be in a car. Take note that 1) I don't mean you should be twitching through your audio tracks while going 70mph on the freeway; and as 2) I was never a fan of having a Discman hanging on a car, as my Dad used to do before he picked up a CD-capable receiver back in the 1990s.
Basically, if the priority is to listen to a lot of digital music of any format, assuming your receiver doesn't have it, it would be most advisable to leave Head-Fi and go over to a car audio forum. Over here any rational explanations of the impracticalities of it can be regarded by some members as a huge irrelevant rant, especially if it's not saying that their favorite portable, personal audio player(which tells you how it's supposed to be used properly) isn't the most awesome thing on the planet. If SQ is a priority, then those online car audio forums really will be your best bet, as they can point you to the equipment and other methods that will deal with the acoustic realities of a car. Even if you sit dead-center because you happen to own a Maclaren F1 or Lamborghini Egoista, you still have to deal with the noisefloor (ie, the engine, the fact that your speakers aren't mounted on cabinets or headphone chassis designed to deal with resonance, etc).