I serviced high-power amps part-time and You do have to understand bridging. Judging from the "DUST", those switches likely need a cleaning.

With no input and no load measure the DC voltage from ground to + spk and - spk and +spk to ground and -spk to ground L&R in bridge and non-bridge mode.

Particularly you get exactly zero between spk- and spk+ in normal mode, I'd look at the output relay. You should at least see a few mv unless the amp is capacitive coupled.

You have DIN and phone plug inputs so concentrate on the phone plugs.

Relays could exist in the input circuit as well. Without a minimum current or
the right relays, the contacts could be oxidized.

One way to bridge is to invert one signal and to take the output from the + spk terminals w/o using ground.

So what I am suggesting is oxidation of relay contacts on the input or output or something between the ears (user error).