1. Possible wrong bulb in brake light bulb holder (single filament) bridging both brake and park wires, trailer plug wiring shorting out or some other short elsewhere in the wiring.
2. connect up that switch wire to your gearbox switch. Don't worry about the other wires (May be aircon related?)
3. that wire goes over the other side of the thermostat housing where there is another temp switch.
4. check the voltage at the alternator B+ post when the car is running, if it is the same as your battery voltage you have issues - Should be around 14v (Could also possibly be a burnt out fusible link).
5. Judging by its position I'd say it is a start relay, Pull the relay out and try start the car, If it doesn't crank, Thats what its for..
Where abouts you from anyway? Put your location in your profile and welcome to rollaclub...
Justin........