Resposta para "Touch function does not work on the bottom of the screen, why?"If the part that you just replaced is the digitizer panel that means you might have faulty digitizer panel. But before jumping to conclusions,i would open the phone again, and i would disconect the cables #1 #2 and #3,and i would take out the lcd+digitizer panel assembly and set it aside. Then i would check with a magnifier mirror,the contacts on the flat cable #1 which is the one responsible for the digitizer panel and then i would take one of those aerosol spray cans,used for cleaning of computer keyboards and i would blow a few jets of air in the conector where the flat cable #1 goes in.That's the special one that you have to lift a small piece of him to be able to put the cable in and then you have to slide it back down to secure the cable in place. It might be a speck of dust or something small inside there that doesn't allow a good contact and then you don't have the bottom of the screen working,like you do now. Good luck.
Resposta para "What and where comes this piece from?"Try to find video tutorials on utube.They have been very helpful to me i the past.You might find one of them that explains better.
Resposta para "iPhone 3G motherboard missing component after the main antenna contact"It is a capacitor and if you don't have that it will influence your wireless reception.