so there is no toll from munich to innsbruck for cars - only trucks?
It depends which way you drive. If you go down the A95 via Garmisch, then you can make it to Innsbruck without paying. If you go down the A8 and subsequently onto the A12 Austrian autobahn, then you do need one.
I wouldn't worry too much about the fine, although it might come. I used to live in Switzerland and often went to Austria on shopping trips. Usually I'd buy a vignette for the shortest period possible, but sometimes I didn't bother and I never got a fine sent to me by mail.
They have only recently started to enforce the Vignette with cameras, so that is probably why you never got a fine in the mail. Previously they had to physically stop you.
To rant a little more about Austria, I have never seen a country out to get you more on the roads than here. Not only do they have this Vignette, but there are also normal tolls. Further to that, the road signs specifically direct you at times through the tolls, even though it would be quicker to just take the normal free roads. Going to Igls near Innsbruck is an example.
Further to that, they have stationary speed camera's everywhere, and in very unfair locations. I got zapped coming out of a 70 zone into a town. After I saw the flash, I went back to see, and the camera was literally 20m after the town sign (no speed sign, just a sign signifying you are entering a town, which means 50). I got the fine and I was only doing 61km/h! Luckily it was only 36 euros, can't complain too much at that.
And then they have the nerve to make you do 100klm/h on their super expensive autobahns! It's a joke. Such a relief once you hit the border back into Germany.