My latest project for the X3 reverse involved switching all the turn signals out to LED lights. I upgraded the X3 reverse lights to LEDs as well, and wrote up the procedure to do it yourself below.
That gap between the tailgate and plastic interior cover is where you need to make your attack
How to get at the lights inside the tailgate of your BMW X3
The biggest factor to accessing the X3 reverse and brake lights in the X3 tailgate is having the right tool. I highly recommend this trim tab removal tool
I purchased on Amazon.com since there are close to a dozen plastic tabs you need to unsnap to access the tailgate lights. I purchased the tool, then opened the tailgate in my garage to a point that made it easy to access the gap between the outer tailgate and interior plastic cover. Read More