New head unit or upgrade?

Hi Guys,

I own a skoda fabia and when i bought this car it came with 2 speakers and 2 tweeters in the front, the dealer fixe a JBL CS-6 or CS-5 for the rear doors, though this was fixed the people in the back complained that the sound quality in the back is not up-to the mark but for the sound quality in the front was of top notch.

So i thought why not get a good tweeter(Pioneer CS-120...) fixed for the rear so when i went to near by audio shop one of the audio guy opened the system and said this actually support the front speakers and the rear speakers were crimped and fixed to the existing ones which i was shocked(later on i also opened to re-check and found the rear speakers were connected to the same wire of the front speaker), he also said there is no point putting tweeters as the head unit may get smoked due to heavy load so now i am thinking about the following and need your help.

1) Exchange the old head unit for some 2 din touch screen system and get a JBL component speakers for the rear section - can anyone direct me on a good head unit, component speaker(model and price) and

a)how much can i expect for the old head unit and JBL speakers my budget for new HU and rear component speakers is about 20K or less,(quote in EUR,$,INR etc, will convert it to get a rough idea)

The component speaker usually has a crossover and then a tweeter, so where is the tweeter connected(Where is it usually placed) if i am doing it for the rear of my car.

2) I was going through few sites and came across an amplifier which takes a 2 channel input and converts to 4 channel which is also a good idea but wanted to know if any one has tried this out.2 channel to 4 channel split system : http://www.scosche.com/car-stereo-output-converter