Just done my 740 but with a non-Volvo lock motor.
Your sytsem`s existing door switch is a negative pulse system.
It grounds one wire with the key turned clockwise and the other on turning it the other way. This triggers the existing c/locking relay to send a pulse down the two lock motor wires which go to all the lock motors to lock the doors.
When the opposite ground is made by the door switch the relay sends another pulse with reverse polarity to the motors to unlock.
I too suggest that you keep the existing relay and use the new system to trigger that.
Your "new" motor wires then just splice into the wires going to any of the existing motors,and those from the remote to the switch wires.
If you can`t figure it out, e-mail me a copy of your remote kit`s instructions
and I`ll try to sort it out.
Colin.