If you're that concerned about this, you should do a little more research on the topic. There are two different kinds of mercury: ethyl mercury and methyl mercury. Thimerosol includes ethyl mercury which is in fact perfectly safe to inject into the muscle tissue. The kidneys are constantly filtering out bad things from the bloodstream, so the miniscule amount of ethyl mercury in an intramuscular innoculation of MMR vaccine containing thimerosol is flushed out of the body (even a tiny child's body) within a matter of hours.
The really bad stuff is methyl mercury, which can seriously damage the central nervous system, particularly when ingested into the digestive tract by, say, eating contaminated seafood. This is slowly aborbed into the bloodstream through the stomach and intestines, where it lingers for a long time. And the more bad fish you eat, the higher the levels of methyl mercury accumulate in the body, and the longer it takes to get rid of, increasing the potential damage to the nervous system.
But neither of these has anything at all to do with autism, which is an ill-defined syndrome, a medical mystery commonly referred to as a "diagnosis of exlusion." This means that when a physician observes certain behaviors and symptoms in a patient that cannot be explained by any other diagnosis, they use the umbrella term "Autism spectrum disorder" which means they don't know what the hell causes it. Over the past few decades, the number of symptoms categorized as characteristic of "autism" has grown enormously, which is why the number of autistic patients appears to have risen. The medical establishment keeps moving the goalposts so that many conditions previously called something else (e.g. "restless leg syndrome") are now lumped under the many hundreds of symptoms called "Autism spectrum disorder."
This doesn't necessarily mean that there are more autistic people in the population now than there have ever been in the past, it just means that doctors are now classifying the sypmtoms differently, which makes the numbers appear to be going up.
Full disclosure: I am not a doctor, but I am married to a nurse, who knows a helluva lot about this kind of stuff because it is her job and she is damned good at it. Ask her about the health care system, ask me to fix your goddamned computer or network.