Display Problem and external display goes black during boot

While trying to replace the keyboard on a MacBook Pro 13 A1278, I might have done something to display.

The system boots up with no display. I hooked up an external monitor via mini display to vga cable and a usb keyboard since the new keyboard I installed is playing up (one column of keys i.e v f r 4 does not work). With external display I get apple logo and the loading bar but half way through back to dreaded black screen.

I have tried every tip I found on the internet. From many many NVRAM resets to trying to boot into safe mode and as well as single user mode.

• Safe mode is unsuccessful since like normal boot half way through booting up external display goes black.
• On single user mode I get the command prompt fine. But existing from it means back to black screen.

I have managed to back up the data and attempted to factory restore via internet but again somewhere along the way during installation I lose the display and back to square one.

I even tried booting Linux Mint via USB, but unfortunately same result, half through installation I lose the display.

One occasion I did manage to get to the login screen on external display, but rushed a bit and removed the external display cable without login in first which resulted in black screen on external display when reconnected the cable.

Getting to the login screen once gave me some hope that GPU on logic board is fine and it could only be display problem. The dilemma I have now is no matter what I do, it goes back to black screen on external display half way through booting.

I was wondering if anyone can give me a tip or two to figure out if the problem is with the logic board despite booting up and external keyboard working and sometime I do get the voice over assistance coming up which itself is pain not being able to turn it off without display.

I don't mind changing the logic board or display if I can isolate the problem but not both since that would a very expensive exercise.