What year is your Maxi?
Is it a straight Maxi or a hard tail or a variant of the standard Maxi?
Why are you certain that the brake light switches haven't been replaced? What color are the plastic tips of the switches? If in doubt order new ones and replace them.
Check the terminal wiring block if it has one (hacked wiring job?)
1. There should be a single Green wire on the engine side of the terminal connecting to, two Blu/Grn wires on the opposite side.
2. One of the Blu/Grn wires goes back to the Brake light and the other goes up to the left hand Brake light switch.
3. There is a crossover Blu/Grn wire that goes over to the right hand Brake switch.
4. Coming out of the Right hand side Brake switch should be a Brown wire going to the Stop/Run kill switch on the right handle bar.
5. The Brown wire goes out the other side of the Stop/Run switch up and winds it's way up into the speedo housing and attaches to one side of the Speedo illuminating bulb.
6. The Brown wire then comes back down out of the Speedo housing and down to the back of the horn where it ataches to one of the Horn contacts.
7. On the back of the tail light assembly the Blu/Grn wire should go to the Brake light (6V 10W) and the Grey wire should go to the Tail light bulb (6V 5W). There should be a thrid wire (Brown) grounded to the frame or fender.