Not a long guide, My Stilo Abarth has been suffering ESP and ASR failures since I bought it. The rear brake lights occationally flashed on, I bought a new brake light switch from Fiat at a cost of £19.99 inc VAT.

To remove the old sensor, first jam a piece of wood or similar between the drivers side seat base and the brake pedal to keep pressure on it. The brake switch is a black and white plastic unit with a 4 point connector on top of it. It points directly upwards above the brake pedal and is a 90degree twist fixing. To remove, it needs a twist in a clockwise direction if you were looking upwards from the cars floor at the switch. When you have spotted the dwitch, unclip the wiring from it by easing the clip up and wriggling the wiring off.

Then use a common plumbers basin tap wrench like this one:
Basin Wrench - Screwfix.com, Where the Trade Buys

(the narrower side) to grip the switch. The jaws slip over the flat sides of the switch. The tool should go onto the switch straight on as if fitted from the drivers seat position. Then twist 90dgrees and fiddle the switch out of its hole.

Refitting the new one is the reverse of removal.

If you turn on the ignition with the brake switch not connected to its wiring you get ASR failure, ESP failure, Gearbox failure and general failure warnings. This switch is needed by lots of systems.

Hope this of help, wish I have photos to show, but the space you work in is so tight you can't see anything.

Andy