It pretty much has to be either the switches, the speaker connectors, or the wiring going to the connections. Because B works, you know the actual power amp, preamp, and tuner work. The light not working is unimportant to the functioning of the speaker switch; most likely, the bulb is simply burned out. But before replacing it, I would get the speaker connections working.
Is there anything visibly wrong with the speaker wire connectors on the back? You will want to look closely, and compare with how the B connections look. Probably, it is either that there is something wrong there, or with the switches. I would recommend that you read up on how to clean the contacts and then try the deoxit again, working the switch repeatedly right after applying the deoxit. If you do not get the deoxit inside the switch on the contacts, it won't do any good, so you will want to read up on how to do this.
Since you will want the case off anyway to properly apply the deoxit, you might as well take a look at the wires connected to the switches and the speaker terminals at the back, to make sure that there is nothing wrong with it. I doubt that you will find a problem with the wiring, because if there were a problem there (which would tend to be rare anyway), it would most likely only cause one of the speakers to not work, not both the right and left speakers.
