Endothes, I would connect up a multimeter across the speaker (if you have an O'scope, that would be even better). Set the multimeter to the VAC scale (5 volts or the nearest on that meter). Now, turn on the stereo and dial it to a mid range volume. Do you see the multimeter jumping around? If yes, repeat on other speakers that are not working, but odds are, you are looking at some bad speakers. If you are getting no movement on the multimeter, then repeat the process on the pins at the back of the radio. If you see movement there, then you have a wiring problem and running new wires is your best bet (stock wires are on the thin side). If you see no movement, then you have a problem internal to the radio and you will be best served getting a new radio (whether a used stock unit or going with an aftermarket and doing the necessary conversions from there).

If in doubt, put the multimeter across the one speaker that does work to get an idea of what a good response from the multimeter looks like. Then you can compare that to the other ones that don't work.

Just for reference, on your back door speakers, the mid bass and tweeters are run in parallel to each other. So, if the tweeter is working but the midbass is not, then that right there confirms a problem with the speaker. The reason why Jag can do that with the rear speakers is that they have a passive cross over on the speaker that limits the sounds it can make. But, in the case of the front speakers, they are powered off of the head unit separately.

if you need the wire colors, let me know. Granted, if your local car stereo shop could not do this test, then they have some pretty weak techs working in the back or they didn't want to take the time to figure it out. Either way, sounds like a bad shop to work with in my opinion.