The P21w is the same size as 1156, its just using 21 watts for the brake. The brake lights are below the turn signal bulbs. The bulbs under the reverse light will only come on when very hard braking and is really designed for rear fog lights as well. I coded the inner rear brake lights as rear fogs and will flash on hard braking. To achieve this you need to code for rear fog lights (must have rear fog light switch), and also brake force coded.
To be honest save your money and buy from Amazon. Get led bulbs that have projector bulb end and these are the ones I got:
https://www.amazon.com/Alla-Lighting...ateway&sr=8-10
When you're ready I'll find the code to code for rear fog setup, but prerequisite is to get light switch that powers headlights, front and rear fogs. Search forum on switch for the x1. I have it but can't remember oem part number. Its a euro part not available in US market