I've been wanting to install a CB radio into my 2006 MINI Cooper S for a while now, and given the current global events I was left with a little bit of spare time to tackle this job. Now, I will warn you that this requires permanent modifications to your car. I'm in no way responsible for any damage you cause to your vehicle. You need to test fit your CB radio to make sure it will fit. MINI explicitly says not to install any aftermarket electrical equipment, including radios. I would recommend that you adhere to that warning if you have any reservations about this DIY. This took me about four hours in total from start to finish using a socket/torx set, plastic pry tools, crimp pliers, small phillips screwdrivers and a magnetic parts tray (highly recommended). Also needed are electrical tape, crimp connectors or a soldering iron, a ground wire clamp, along with the radio and antenna of your choice. I also recommend picking up an SWR meter and coax cable jumper to fine tune your antenna height.

The biggest variation between installs will most likely be the antenna. I chose to go with a glass-mount antenna attached to the windshield. If you never had to drive into parking garages, I'm certain you could affix a 102" whip antenna somewhere, but a smaller mag mount or through glass antenna like mine will do the job (with limited range).

If you chose to install your antenna in a similar location, you will have to run the coax cable behind the interior A pillar trim for a clean install. The trim simply pries off, but be careful as it's attached by two ropes that hold it in place should the airbags ever deploy. You have to be very careful when routing the coax cable, as you don't want to interfere with the side airbag. I routed the coax cable behind the airbag strap.
Once the coax is routed from your antenna, pull off your weatherstripping and the side panel for the dash using a plastic pry tool.
Remove the glovebox with T20 screws, one located in the far back, two on the sides and two on the top.
Disconnect the glovebox light connector before removing completely.
Now route your coax cable above the glovebox and into the compartment behind the radio stack. I chose to coil up the excess coax cable up above the glovebox, but ideally you would use a shorter coax cable.
Pry down hard on the trim underneath the steering column.
Unbolt the four torx bolts to loosen both the downtubes.
Remove your cup holder covers/ashtray on the forward center console, unscrewing the phillips head screws underneath.
Pry up on the mirror adjuster to reveal two phillips head screws, remove them without disconnecting the mirror adjuster.
Pry up on your shift boot to disconnect it from the center console.
Remove the T20 bolt holding onto the rubber ashtray in front of the cupholders, and feel up to release the top part of the ashtray from the radio/HVAC stack.
Now maneuver the forward center console out of the way. You do not need to remove it completely, as we only need to gain access to the 12V power going to the cigarette lighter.
Your downtubes will slide out from the bottom, set them aside for reinstallation.
Now, you should be at this point. Disconnect the red and yellow wire from the cigarette lighter (slightly different in my car as I added USB outlets). This will be the main power supply to the CB radio.

Using a scotchlok connector or soldering iron, take the red power wire coming out of your CB radio and splice it together with the red and yellow wire. Cut off the excess and tape off the end with electrical tape.
The chassis ground is located to the left of the stereo, I will be using the large torx bolt as a grounding point. Crimp on the ground wire strap and reattach the torx bolt to the same location. You can see I already had a ground strap for my hardwired dash camera, in a standard car it would be a bare screw.

At this point, after test fitting the radio I realized the Uniden PRO505XL was quite a bit deeper than I was expecting (it helps to look at the manufacturers specifications before purchasing). I made a modification to the rubber ashtray in which I cut off the top flap from the ashtray. I would say this step is required for all CB radios, if you decide to use the same mounting position.

An optional step is to sand off the hard edge to smooth it out and make it look cleaner. If you decide to remove the CB radio you would have to order a new ashtray, or just live with it.
Now, connect your coax cable you have routed above the glovebox and to the radio stack to the CB.
Test fit your CB radio again with the bracket attached and mark with a Sharpie where you want the bracket to go. The best place I could find is just behind the switch panel. (Please do NOT screw holes into the switch panel itself!)

Here with the switch panel removed (not required, but you should definitely double check and make sure there aren't any wires resting on where you intent to attach the bracket), you can see where the screws are attaching to. They do not interfere with the switch panel at all, and it's a very solid piece of plastic.
I will say the toughest part of this DIY is getting the screws started into the plastic. You have to push up hard while screwing into the plastic, which takes some time and patience. I used a stubby phillips head screwdriver to get it started, then switched to a ratchet with a screwdriver attachment once I got it started into the plastic.
I tried to find a way to remove this piece from the car and drill it out on the workbench, but I'm pretty sure you'd have to tear half the dash apart to do this. So it's best to do it the hard way and use your hand. There isn't any room to get a drill in there with that piece installed.
Now we're on the tail end of the DIY. Connect your coax cable and test your CB radio with the ignition on.
With the CB radio working, turn the ignition back off and install the modified ashtray with the T20 screw. Reinstall your downtubes into the center console, from below.
Now reinstall your center console, installing the four silver phillips head screws into the forward cupholders and underneath the mirror position switch.
Push in your mirror position switch.
Route the wires behind the CB up behind the radio stack and install the CB into the bracket using the included thumb screws.

Reattach the four large torx bolts that hold the downtubes in place.
Reinstall your glovebox using the five T20 screws.
Snap the trim underneath the steering column back into place, this sometimes takes quite a bit of force.
Finally, I decided to mount the mic on a magnetic phone mount I installed on the dash trim. Simply attach the steel plate to the back of the mic and place it onto the magnet.

And done! At this point I would use an SWR meter and tune your antenna. There are plenty of guides online for how to do this. Even before tuning my antenna I was able to pick up truckers at the quarry a couple miles from my house. I'm currently waiting on an SWR meter to get the antenna dialed in for a real test, but I expect it to perform well. You could certainly get more range with a 102" whip antenna but even I have limits on what I'd want to do to my car.
This guide should give you a good idea of how to install a CB or really any aftermarket radio into your MINI Cooper. You don't have to mount your radio underneath the switch panel, you could mount it anywhere you want. Same goes with the antenna. It's entirely up to your preference, but I will say I'm very happy with how this install turned out.