I have just done this modification on my 740. The instructions with the kit simply showed two wires as out-puts to the existing lock/unlock system.
I think that many people interpret this as meaning that the door lock motors can be fed directly from the electronic control module which indeed, they can, but personally, based on experience, I have grave reservations about the ability of cheap electronics to switch relatively high powered inductive loads and wired mine as you suggest,thus limiting the current imposed to that of the existing C/L relay coil which, although I have no definite figures I would expect to be less than 150ma.
I am sure that your burned-out unit was caused by overloading the electronics. Five motors running simultaneously and then momentarily being stalled as they reach the limit of their travel are bound to draw much more than 300ma.
In short, you have it right,and there is no need for supplementary relays to back up the original.
Colin.