I'm not a shop owner but I've heard this before. Someone wants to make a living out of doing car audio installs, then they open a shop and never have time to work on cars. Everyone I've come across who does the 1-man install business is doing it as a side job. Like many industries the work isn't usually steady year round, so you have to diversify even if it means doing things you don't enjoy.
I haven't gone the typical route of starting as the shop ***** and working my way up, but to me it sounds like you'd be setting yourself up for a few years of penny pinching if that's your only source of income. Don't open a new shop if you are a first time business owner. At the very least take online courses for entrepreneurship, business accounting, maybe a management course, intro to marketing and advertising, and so on, while you keep your day job.