The Saber-class does have glowing impulse engines, though that is probably because the model was remade when used for the later seasons of DS9. You can see them in aft view image of the ship on this page: Saber-class CGI.
Interesting. I always recalled the Saber having rear impulse engines in that location.
But anyways, back to the Steamrunner. It should have the impulse engines in those square-ish notches in the back of the saucer. Dunno why they frakked it up though.
I'm going off about the Nebula because its the only other vessel I know of that doesn't have visible impulse engines. In any other case, the Steamrunner shouldn't have any impulse engines to begin with if we go by on screen footage.