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When installing a similar system in my 240s (I've done a couple), I run an extra 2-wire harness, usually the wires pulled from a passenger door lock, all the way through the hinge area rubber boots and over to some point where I can connect to the plug for either rear doors or the passenger door- whichever is easier.
This powers the new lock motor in parallel with the existing lock motors- whenever the lock relays are triggered, the original power lock motors will operate, and the new one as well.
The locks are triggered by tapping into the wiring that already exists for the driver's door key switch. In the 240, this is also in parallel with the lock-knob switch. The arrangement is a bit different in the 740, but the basic circuit and operation is identical: the door switches ground either the red wire (unlock) or the green wire (lock), and that actuates one relay or the other.
The remote keyless install requires connecting your new keyless controller box to the lock switch circuit wires. The box grounds the appropriate wire when the remote button is pressed; the relays cycle to apply +12V to either the blue or yellow lock motor wire; and all the doors activate at the same time.
I've seen no indication that the lock relays object to the 20% increase in current by adding one more lock motor and my installs have worked perfectly for me.
If there is an additional pair of wires available in the driver's door, due to unused power mirror wiring, that would be a great thing. In my wife's 90 745, which is about to get a remote keyless install, I will be installing power mirrors from a parts car at the same time; I believe there will be no spare wires. The plugs to the mirrors are circular 4-pin connectors, though, so it sounds like you've identified the right connectors if you want to go this route. Personally I find it tedious to pull wires through the boots, but not impossible by any means. I cut off the mating connectors and a bit of wiring harness whenever I'm acquiring parts like this from a parts car- makes life easier when it comes to splicing things in.
My remote keyless installs have all been 100% "add-on". The idea is, do not modify anything in the car's wiring harnesses- instead make up little plug-in harnesses of my own with a male connector, female connector, and wires spliced in to go to the keyless controller. In the 240, the door lock switch wires go to connectors that look like this:
Good luck with the project- I'll post back about how my own 740 install turns out too.
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::: Rob Bareiss, New London CT ::: 92 244 M47 211K ::: 90 745GL M47 273K ::: 88 245DL AW70 190K ::: 84 242DL Project ::: 70 VW Bus ::: 70 VW Pickup Project ::: 71 VW Notchback :::
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