Scared of where litter box is

I have a very nervous 3 year old tabby cat.  No idea why, he's always been like this. Maybe because he spent so much time in the shelter before we got him?  

Anyways, got a litter box problem.  We've had them out in the garage, no problem at all, the cats have a door that they can go in/out of.  There's stuff in there that has never bothered him.  In fact, we had a table/chairs from our daughter's old apartment in there until last week and they had no problem. 

Well- the neighbor gave us his old riding lawnmower.  Ok, not gave, it's costing us a whole $80 to have a lawnmower that's 10 years younger than the push mower we currently have, and it's in very good condition.  Thing is, the "thing" in the garage is scaring poor Sam.  To the point where he runs out of the garage.  Litter box wasn't working in there, so I moved one of them into the house.  Problem is, husband doesn't like it there.  Well- I don't like cleaning up poop and pee, or having sick kitty. 

Eventually I'd like to move it back in there.  Any tips on how to get the animals to get used to the "scary thing" in the garage?  I think part of it is that they've got a dog and I'll bet the cats can smell that.  I was thinking about putting one of the pheremone things I have in there.  Opening up the garage door when I'm home so they can roam in/out freely is another idea I was thinking of if it's cool enough.  I put a few treats in there and was able to get one of them (the less nervous one) to eat a few treats in there.  

Thanks! 

Cheryl

@Sneakymom

Since you suspect that the lawn mower is freaking out your Sam, why don't you 1st give the mower a good wash to rid of at least some smell of other cats and dogs if there is.
Then have a plank just to block out the view to Sam, yes, he knows its there but at least it's not staring him in the face. Or move the litter box to another location in the garage?
Another thing is carry Sam and walk over to the mower and sit on it and let him get use to it and of course don't start the mower in the garage if he's in there. That will freak Sam out more.
When I bought a new vacuum cleaner, my boy was freaked out too so I left the vacuum cleaner all over the house just to let him get accustomed to it bi bought a new one because the old one was very loud and the sound and looks freaked my boy out.
So with this new one, when I use it, initially I just tell him, it's ok, it's a vacuum cleaner. Somehow, he gotten use to this new one and he doesn't run away but he stayed close to it.
So maybe you can try to build up Sam's confident over this mower.