The keyboard on the Commodore plus/4 is connected via an unusual flat flex cable with thin wires in. These don't tend to last well, they are folded over from the keyboard and press against parts on the back of the keyboard and the mainboard below. They can also tear off easily if you don't open the case carefully.
Normally when I make a Commodore plus/4 USB keyboard, I start with one that has a broken flat flex, and replace that with a pin header and plug on the USB keyboard controller.
I've been asked for a Commodore plus/4 USB keyboard kit, so here is a version that can be used on a plus/4 that has a working keyboard and an intact flat flex (and most likely a faulty TED or 8501).
The USB keyboard controller sits on the back of the keyboard and the flex slides into the connector on the edge of the board. This one had metal tape added to protect the flat fex, which is probably why it is still intact.
The USB cable plugs into the board and out the back of the case.
This version has a fixed keyboard mapping, so all the keys mapped appropriately for the normal desktop use. I do a version with a mode switch for emulator use, but that requires a bit more modification and changing the LED etc., so that is not available as a kit.