Car audio doesnt pay that great, you deffinatly cant make a living off of it unless you own a shop. Being a a certified mechanic at say a MB or BMW dealer pays way way better (tripple digits usualy).
Really depends on how good you are, and what kind of demographic you are surrounded by.
If you work at a custom shop in a low-class area, of course the money isn't there. What about a hi-fi shop in a upper class area? I know of installers that pull in triple digit numbers.
It is true that there isn't much money in installations- but it CAN be had.