Then there is factory amplification that you are going to have to tap into. My lexus is this way and I bought a Kenwood DNX7100. I went to Sound Advice (Tweeter) to just look at it and for kicks I asked them how much they would charge to install it.... since there is no dash kit for my car, they would have to fabricate something; there are dash kits for the new mustangs though. They told me they couldn't tap into the factory amp and that I would just have to run the door speakers of the new HU and lose the factory sub. Basically what I'm getting at (and every car is different, mine sucks for this crap) IF there is a factory amplification interface you can buy, then it might not be so bad, but you'll still probably have to run RCA's which means your cost of install just went up. So, all things considered you could possibly be looking at Cost: Dash kit, rca's, 2 wire harnesses, maybe an adaptor, and more billable time taking apart the car to run the RCAs depending on where the factory amps are located, which sometimes they are on a rack in the trunk. They wanted $400 just to install the HU and bypass the factory amp, becase what they said they were going to do is run a bundle of 4 speaker wires from the HU to the trunk where the factory amp is and splice into those to feed the speakers. Hopefully yours is not the same way... I'm somewhat unfamiliar with the 08 Mustang stock system. Just guessing from older mustangs, i.e. the Mach 1000 which basically had a huge amp rack in the trunk already.
Anyway, add that to your BT and ipod, and my advice is, wait till you have everything else, i.e. amps, speakers, etc because the install cost in the end will be less. Or, if your car is this way like mine is, if you're going to have to have all that run just to do the headunit, have them run extra RCA's and power wire under everything as well while they have the car apart; they can hook up the RCAs as they should, and label them, so that when you are ready to do the rest you can just plug everything in. Maybe you don't want to do anything else with the car except this, and that's fine, but its an addicting hobby and youll find yourself wanting more.