I can't help with specifics of the FP-8, but there are probably some screws along the back and on the bottom (along the sides of the bottom, not the ones holding the keyboard assembly down) which will allow you to open the case and have a look. This post
might be useful.
It would be nice if digital piano manufacturers made their products more repair-friendly. While they're designed to not need servicing, parts obviously do wear out and fail after a few years.
They should make
- repair instructions widely available
- the pianos themselves easy to disassemble without tens of screws and a minefield of fragile connectors
- the parts accessible and easy to replace (desoldering and soldering in microswitches is risky and not easy)
- parts available with cheap delivery via an eBay or Amazon distributor (I ordered some switches costing pennies from Roland and the handling was £4.79 - they're probably not overcharging, but just don't have the economies of scale or efficiency that other distributors can achieve)