When Iago stabs Roderigo in Act 5, where does he stab him?

The text doesn't actually tell us. We know that Iago wounds Cassio in the leg from behind, when Roderigo fails to kill him:

ROD:
I know his gait; 'tis he. Villain, thou diest!

CASSIO:
That thrust had been mine enemy indeed,
But that my coat is better than thou know'st;
I will make proof of thine.

ROD:
O, I am slain! 
[Iago from behind wounds Cassio in the leg, and exit.]
 
CASSIO:
I am maim'd forever. Help, ho! Murder! Murder! 

Roderigo is wrong, though. He isn't slain. He lies on the floor crying out for help, but no-one dares come in case he's a faker trying to set a trap so he can hurt them:

ROD:
O wretched villain!

LODOVICO:
Two or three groan; it is a heavy night:
These may be counterfeits; let's think't unsafe
To come in to the cry without more help.

ROD:
Nobody come? Then shall I bleed to death.

Roderigo lies there in the dark. Until Iago steps in with a light to see what is going on - and finishes the job:

IAGO:
O treacherous villains!
What are you there? Come in and give some help.

ROD:
O, help me here!

CASSIO:
That's one of them.

IAGO:
O murderous slave! O villain!

ROD:
O damn'd Iago! O inhuman dog! 

And that, stabbed in the dark, is the end of Roderigo.