Tried to install apple carplay. After installing receiver harness behind the stereo, lost all audio and hazard flasher malfunction warning message appeared. Also no power to heated seats buttons and eco button. Guessing I broke some wires in original harness somewhere along with fiber optics cable. Any ideas which harness I should be looking at? I removed apple CarPlay and put all original equipment back in place. Still no audio and power to buttons. No obvious damage to wires in immediate area.

 

I'm going to take a guess and say maybe you smashed a wire putting the radio back in and blew a fuse somewhere. I don't think it has anything to do with the fiber cable cuz that wouldn't cause the power issue as it only feeds the audio signal to the amp from the head unit. Check all the fuses with a test light or volt meter see if you find a blown one. Couldn't hurt to remove the tape and inspect the wires in the harnesses behind the head unit also.
If you find it then test everything with it all lose to see if it works. If so install your upgraded screen and test again before cramming all the stuff into the dash and test after cramming stuff into the dash.

When I did mine the cable management and getting everything to fit back in was the hardest part of you wanna call it hard. There was room on my 2014 c300 to the right of three head unit but just left off the glove box to fish the new screen harness up to the top. Then all you have to deal with use the harness adapter on the rear of the head unit fitting back into the dash.

Hope it's a easy fix for ya