Home Button & Microphone not working - Help appreciated!

Hello guys!

I am totally new to this forum, but I hope you can help me anyways!

Some time ago, I bought an old iPhone 3GS from a friend. It was not in a very good condition so I ordered a new back plate, with all new buttons, battery, vibrator, microphone and so on. I changed it and my iPhone was fine for a while, untill I dropped it and broke the screen, which went into the "white screen of doom mode" or whatever you guys call it. I bought a new screen, replaced it and now the home button only works sometimes, same for the microphone. It usually helps to pop the screen up and back down, but then after a while it stops working. I can't charge or connect it to my computer either, so I guess theres something really wrong with the bottom part of the phone..

Is there something you think I can do to fix it, cause I have used quite alot of money on this phone and don't feel like ordering a whole new back panel when it should be as good as new.

Ps: I have the old back panel which I guess I could take some spare parts from if needed.

Thanks in advance for reading this!