If your on a budget then you might want to plan your system in stages. I'd start with the deck (head unit). My personal preference is Alpine but there are a lot of good manufacturers out there. Make sure you get something with high power, for instance my deck puts out 27w per channel RMS and 60w peak. This will make your current spkrs sound better until you can afford nicer ones. Next you want a deck with at least one set of pre-amp outputs if not more. One set will do you if you buy amps with built in crossovers that can be linked together (daisey chaining), but you will need seperate outputs for front, rear and subwoofer if your amps are more basic. If you've never installed your own deck before most audio shops offer free installation when you purchase a deck from them. If you do it yourself make sure you run a good sized power wire straight to the battery (like 8g) and also from the ground to the firewall. I like to use "bigger then called for" spkr wire too, it has always paid off for me in sound quality. Another good idea is to install sound deadening material like Dynamat extreme to the surface your spkrs are mounted too. You can get their spkr kits or small sheets dirt cheap
online and the difference can be amazing. Most these guys will tell you the installation is the most important part. Bargin bin parts installed right will always blow away high end stuff installed incorrectly or half hazardly.
Replacing your spkrs would be my next step. Your Mustang has 6x8's in the front doors and either 5.25's in the rear or 6x8's depending on the system originally ordered. You can find speakers to directly replace these which would be the easiest and cheapest; or you can save your money and buy a component system for the front (rear spkrs are generally just for fill so decent 2 way or 3 way spkrs will suffice there). That all depends on how involved you want this process to get. There are a lot of good "how-to" articles online so if you've got some tools, don't be intimidated. Whatever you do, just don't forget the sound deadening material. Just doing the areas between the spkr and the metal it's mounted to for about a 6" diameter around the spkr and then covering any holes between the trunk and the inside of the car and/or between the inside of the doors and the car can make huge improvements in bass, sound quality and volume levels.
Lastly I would buy a sub(s) and an amp(s). Here is where it can get expensive. You can buy a powered sub if you want hassle free installation, a sub in a pre-fabbed enclosure w/ a mono amp to run it, or you can custom build your own box. I'd go with a loaded pre-fab box and a mono amp.
MTX makes an enclosure designed to fit your car, but you can find more generic boxes for less money. I'd come back when you get to this step cuz there is so much to consider and learn in this area whole books could be written (and have been).
Hope this helps you get started. I'm sure some of these other fine audio freaks will fill in anything I left out. Take care and God bless.
