In all the walkthroughs it just sais "the beginning of the map", but I can't find it there or anywhere. Ended up getting killed while looking, and I'm playing tough guy, so don't want to start again until I know I can finnish.

It's where you start but over to the left side of the building you start in.

It's http://www.project610.com/files/St_Louis_fbos.jpg it's the cycle maked in red with 4.

Thanks, seems you have to drive on that area before the grid actually appears, lame. Next question. Is there any way to completely disable dangerous enemies? Blinding, concussing or crippling an arm doesn't seem to have any effect, if you knock them out they can wake up again, and killing them off with sniper can take quite a while.

What exactly do you mean by disabling dangerous enemies?

If you're having a tough time fighting the Super Mutants, invest a lot into Big Guns, they are the way to go against these opponents. St. Louis can be a very difficult mission if you only have Small Guns specialists, it takes a long time to pick them off with the guns you have right then. Since I played through the game multiple times I prepare my main character and at least two others for this mission by raising Big Guns before getting to this mission, that way it's a lot easier. Bring an explosive specialist for the mines on the roads, a sneaky scout (preferably with a high Big Guns skill or a good shotgun) and keep most of your team inside the armored vehicle. Most Mutants are difficult to flank in this mission, so picking them off at a distance seems tedious but it can be the best method if you don't want your team to suffer casualties.

Since most of these Mutants use Big Guns, crippling the arms should have the effect that they won't be able to use their desired guns anymore. If that's not happening in your game, maybe you should try patching your game up to the latest version if you haven't already. Otherwise, I have no clue why they can use their Big Guns on you with crippled arms.

You have to cripple both arms?

One would have to suffice, since the Mutant needs both arms to properly hold, aim and shoot a Big Gun.