All that up-and-down converting to simply play music in the car is sort of like going 'round your arse to scratch your elbow, isn't it?
Ditch the converter and wall plug and power it with 12 VDC. If you have time to do all that fiddly nonsense with the converter/wall wart, you can surely find time to simplify it.
And, yes, if the system is loading that much with the converter/power supply/amp that it dims the lights or even kills the engine, the alternator is on it's last legs. Get it tested BEFORE it strands you. Battery, too. Get it load tested, not simply voltage tested. There is a difference.
Quotes to live by:
guessing gets expensive
nothing is more expensive than a cheap BMW
buying a ratty example (of a BMW) is a parasitic relationship.(and you ain't the mosquito)
Ken Kanne, Silverhill, AL, E36 Forum Mod/Craigslist addict/Hoarder of all sorts of stuff
1995 318is "Bebe"; 1993 325is "Elvira"; 1985 635CSi "Katja"; 1984 633CSi "Sylvia"; 1987 325is "Odette"