Just because it is a new switch doesn't mean it works!! (I'm not joking either).
If you still have the old switch, test it. As I said earlier, I had a peculiar problem with mine and all that was needed was to reset the brake switch - push the pedal down then pull the plunger out and release the pedal. There are two types of brake switch for the Classic IIRC and the difference is a subtle one, I'm a bit rusty on it now but it was something silly to do with the way the switch is adjusted but also remember it is a two stage switch, I can't remember which way round it is but one set of contacts is only for the brake lights, the other set is connected to the ABS ECU. I believe any ground in the circuit is through the brake lamps as the switching is only for +12v feed rather than being for making or breaking ground.
After a quick review of the ABS schematic, I think I understand your reference to a 5 amp fuse. These fuses are located under the passenger seat. You need to remove the lower side plastic trim from the seat base, this should expose some relays and fuse holders, in here you will find all sorts of potential gremlins so do have a careful look, especially check for corroded wires/connectors and also check for earth wires (normally black), make sure all are clean and secure.
You should see 3 relays - ABS warning light relay, Valve relay and pump relay. 4 fuses - pump relay, stop light switch & diagnostic plug, valve relay and then the big 30A maxi fuse for the pump.
If you're messing about getting blink codes from the ABS ECU make sure you have properly cleared down all stored codes, this can take an annoyingly long time of faffing about!! but also make sure you disconnect the jumper wire and put the relay back.
Let us know how you get on, it could be it was just the brake light switch and a replacement sorts it, but if not I am afraid you'll need to be getting busy with the multi-meter.
