I read that in the US the airlines and manufacturers have started a program to help "other trades" mechanics become airplane fixers. (Here in Canada the government has the usual stunned response. We can expect action about the time hell freezes over.)
Since you're already a motor vehicle mechanic you already have the tools and skills you need, but you'll have to prove it somwhere. GA aircraft are far less complicated than autos, the hardware is of much better quality, rust doesn't fall in your eyes and the clientele are more likely to believe what you say. Your back won't hurt as much, either.
But: Pay scales are low, hours can be long.
I tried working for an airline-like outfit for about 6 months. I hated it. I was mechanic (lowly peon) #118. If I needed lockwire, a bolt, more rags, anything, I had to stand in line to requisition it.
Better to work for small outfit but NOT fixing private owners aircraft. Corporate aviation might be good. I'm very happy working for a geophysical survey company engineer/pilot. Small teams, good pay, travel. Boss genuinely concerned about safety.
Good luck.