Here's the electrical diagram including the brake switches and the mechanical switch I added to the brake light circuit:
Basically I put a mechanical switch between S1 and S3 (82a and 81 in the diagram) and left the ground/"check brake light" (S2, 81a) connected to the pressure switch. So if you jumped S1 and S3, the brake lights should come on.
Make sure the wire that goes to S2 is black in colour.