Been an electronics and audio guy for about 15 years but this problem has me stumped so I thought I'd reach out to you all for some ideas.
I have a 2004 Infiniti G35 that is pre-bluetooth connectivity and I wanted to get it to have at the very least bluetooth streaming music from my iphone. What I did was buy a cassette adapter and then connect the 3.5mm cable coming out of the cassette adapter into a bluetooth adapter, which would connect the my phone. It was all working fine but I took my car to get some minor electronics issues fixed and the tech suggested that i have an RCA soldered directly onto the back of the PCB for the stereo so that it would be a cleaner sound. I said fine, cause it was only like $20 extra.
Anyway, I get the car back and when I plug my phone directly into the cable they soldered for me it works great. Sounds awesome. now here is the kicker... when i plug that cable into my bluetooth adapter and try to play music through the phone it sounds all garbled. When i take the bluetooth adapter and use it with another stereo system (like my home stereo) it works fine. I can't figure out what to do...
I thought it may be an issue with the 3.5mm jack not connecting with the inside of the bluetooth adapter so I got an extension cord to see if it was the connection. Same garbled sound. I am out of ideas.
It could be that the output gain level of the bluetooth adapter is just beyond the level that the input stage of the car stereo can handle without distorting. If there was a way to decrease that gain (even temporarily), you could test that theory (an inline fixed or variable resistor, for instance). Chances are the home stereo has a wider range of allowable input voltage, and thus doesn't distort.