Ok, some facts for you.
First:
JBL system. Includes head unit (+ optional CD player), 4 channel high pass amplifier which powers 4-4" component speakers with passive crossovers and 1" silk dome tweeters in 6x8 enclosures, a mono amp connected to a 6.5" subwoofer in a sealed enclosure. Power ratings were not given for these amplifiers, but the system was originally advertised as a 200-240 watt system. Logic would tell you that could be divided up as 100/4 for the mids and 100 for sub, or 120/4 for mids and 120 for sub. Either way those had to be some sort of max rating as the system has in fact been tested and documented at 12.5 rms/channel for the mids and 35-45 rms for the sub amp. Certainly not worth bothering with unless you just happen to come across the stuff free or really cheap. Consider that the many newer head units with the Mosfet chip in them claim 22 rms/channel.
For those who wish to use parts of this system:
The entire low side of the system (inputs) are a voltage divided floating (internally bridged) mid/low level input. The system will accept any audio input, however the results will vary from downright lousy to pretty decent. If you send in an RCA level input, you will not achieve full volume as the voltage levels are just too low. If you run speaker level inputs from an aftermarket deck you will meet the floating inputs requirement, but you will not be meeting the carrying voltage requirements and will see a lot of noise and background hiss because of this. You will also be overdriving the inputs causing distortion, screwing up the crossover frequency and risking damage to the amps. There are a number of adapters made for the purpose of making the outputs of an aftermarket deck fully compatable with the system. The best of these by far is the Install Edge adapter from Rockford Fosgate. They run about $40.
So that is the complication with running an aftermarket head source. As for the speakers themselves, they are only 4", so I would not run a full range signal to them from an aftermarket radio. If you are using an aftermarket radio or amp without the JBL amps I would most certainly make sure you have a 125hx crossover on the main speakers. Either way you cut it you really need a sub to get decent sound from the JBL speakers.
Regarding the subwoofer, I don't know if Ford even offers a replacement anymore, but I do know that the Explorer has a 6.5" sub in it as well and they are about $65 from the dealer. Less if you have connections. I don't see why this wouldn't work. Some people have also modified the enclosure to accept an 8" aftermarket sub. Word is that this works really well.
I have also used the factory sub to power an aftermarket 10 and 12" sub mounted free air to the rear deck. This works pretty good also if you don't have the original enclosure or don't want to replace the blown stock speaker.
A regular premium sound amp will work in a JBL car if you have no sub anymore, but expect pretty poor sound for the reasons listed above.
As for adding JBL components to a non premium sound car, forget it, it's just not worth it unless you have a subwoofer already and just want to use the 6x8 enclosure speakers. Nothing else is useful to you.
If you do have premium sound already, all you really have to do is plug in the amps. You will need to run a 10ga power wire from the battery for the sub amp though. Remember though that the premium sound speakers are only paper dual cone 6x8's so the JBL speakers would be a big plus here.
That's about it. With the low cost of aftermarket stuff these days though, I wouldn't bother to mess with it unless you are going after a restoration type thing.