Batboard

I have a old autoradio (cassette and not AUX input ) in my car. But i don't want change because it pretty. I just want to add bluetooth. I leave FM
receiver side for the true audio / radio.

I want to know if i purchase both parts :

TDA7388 four-channel 41W audio amplifier board and
Bluetooth home-car receiver adaptor
I connect the audio amplifier with bluetooh receiver and power amplifier with speaker and i add a switch, to switch(speakers) between true audioradio/power amplifier.

I know i must add a power convert 12V->5V for the Bluetooth module.

I want to know if this is good "cheap" solution ?

have you looked into a Parrot kit?

https://www.parrot.com/ca/car-kits#Hightlight

They make wiring harnesses that interface into almost any vehicle radio system, to give you Bluetooth phone & music audio through your car radio speakers. They auto-mute the radio when using the phone as well. We sold a ton of these over the last 8/9 years. Not any more with built-in bluetooth becoming standard in nearly every vehicle now.