Front shifters work great. Rear shifter doesn't respond.
So I popped open the rear shifter and guess what I found:
Rust and corrosion everywhere but mainly around the receiver section.
The internals are covered by a metal shield so I'll have to desolder the shield to see what's else is messed up.
The display module did not indicate any malfunction but it's obvious now why you should tape your heads.
I'll post more pics of this repair when I can get my hands on a soldering iron and solder remover.