I hope is it not too late to share this information with FC owners here.
After my car was fitted in the front and rear dashcam, at the car accessories shop, the FM radio signal became very bad. Only can listen to a few channels.
Recently, I decided to diagnose the problem. I noticed that when my car is in accessory mode, the radio signal is perfect. When I start the engine, the radio signal becomes bad again. After a few examinations, I ruled out LED bulbs (High, Low beam, fog lamp) causing the problem.
I went on to check the fuse extender connect with a 12v USB car charger that powered my dashcams, installed by the car accessories shop.
The USB charger was provided by me (wanted to save money, and I have a few spare).
The shop just help me to connect the charger to the fuse box using a kit. Refer to the picture below.

The USB charger has 2 USB ports. As soon as I disconnected 1 of the USB cable. The radio signal interference become must less. If I disconnected both USB cables.
The interference is completely gone. So I have found the root cause. To try to solve it, I replace the problematic charger with a few spare car chargers that I have.
I noticed different brands/quality/prices of chargers produce different results.
Below are the ranking according to the interference produced by my chargers:
No interference:
-
Anker (1 USB-A, 1 USB-C ) . The USB-A didn't produce interference. not sure about USB-C
-
No brand (1 USB-A). The power is not stable, dashcam keeps on turning on and off

Little interference:
-
ROCK (1 USB-A)
-
No brand (2 USB-A). When connect with 2 USBs, full of interference

A lot of interference:
-
LDNIO (2 USB-A) 
-
70mai (1 USB-A). Comes with my dashcam

-
No brand (2 USB-A) 
In the end, I use the ROCK charger connected with a fuse extender to power my rear dashcam. The front dashcam I reroute it to the left-hand side through A piller then go down to the glove box and connect to the Anker charger which is plugged into the cigarette socket.
The reason I don't connect the Anker charger with the fuse extender is that it is fast charging and supports PD. Is a waste for it to power just the rear dash cam.
Although now still have a little interference, it is very acceptable. Compare to before, I totally cannot listen to 88.1, is all white noise. Now I can listen to it clearly and occasionally some white noise, depending on location.
Note: My fuse extender is connected to fuse #3. This fuse only will have power when the engine is started. To find out where is fuse #3, please refer to the fuse box label next to the fuse box or refer to your owner's manual book.
If you want to buy a new charger to replace the existing one, I advise you to buy a quality one. It may cost more, but I believe it will give fewer problems. There is a saying buy cheap, buy twice.
I hope my experience is able to help someone who is also facing FM radio signal interference.
Reminder: For safety reasons, always turn off your car engine before you touch any wires or the fuse box to avoid accidentally frying your car's electronic components or even your fingers. If you have no experience in DIY your car, I recommend you bring your car to a car accessories shop and explain the situation to them, pay them to do the job for you. What's the reason u don't consider using hard wire? I route it myself & lots of tutorial on it on YouTube if u are good at DIY