So, armed with the excuse that I was "saving the vintage lock cylinders from wear and tear," I cobbled together a power lock system from E30 parts.
It's as basic as I can make it. I discarded the E30's somewhat complex central locking electronics and boiled the system down to a few simple components: door solenoids, a couple of 5-prong relays, and a switch. Piggy-backed on that is a $15 eBay remote keyless entry module, which triggers the lock and unlock function. Powering the system is a $10 auto parts store fused power block, which also allows for future powered additions in the rear of the car. This one is getting constant power from the battery. I have an identical fused power distribution panel under the dash, which is is fed by a relay to come on with the ignition.
Here is the wiring diagram. Basically, you need to power the solenoids one way to lock and reverse the polarity on them to unlock. 5-prong relays with the 87a terminal are the key:
The remote keyless entry unit is wired to ground the 85 circuits.
Once that was all together, I ran the wire pairs, including the trigger pair for the interior lock/unlock switch, to the cabin. In-wall, insulated speaker wire is nice to use for this, since each pair is enclosed in its own insulation. I grabbed two E30 door lock solenoids from the salvage yard for about $9/each (*If you get them, remember to keep the mounting screws) and bolted them into the doors. If you clip off enough of the harness from the E30 door, you will save the amount of wiring necessary for the doors in your 2002.
I just used a few grommets and positioned the wire bundles to slip through. Just make sure that you secure the wiring every 6-10".
No, I did not chop those giant, ugly speaker hole in my doors.
I bought a stick of 1/8" steel rod from the local hardware store and bent it to fit. You might screw this up the first time around, so be careful.
You'll need to drill an 1/8" hole in the door lock mechanism. Go ahead and oil it up while you're in there.
You will also need to drill the solenoid to 1/8".
Once it's all installed, you can enjoy simple, non-invasive, and mostly removable electric locks with keyless entry. Your days of walking all the way around to lock you passenger door are over.