Hmm, I really like your 9-volt battery idea---never thought of that before.
Only the doorlock key cylinders will be shaved, correct? The handles to actually open the door will still be as factory?
I have two ideas of my own, as well a a suggestion about doing your 9-volt battery thing.
--You could simply install wires from the battery terminals, going to some location under the car, and then use jumper cables from another car to power up the Integra.
If you go to junkyards, take a look at some of the cars that have the battery mounted in a remote location---they nearly always have jump-starting terminals under the hood. Examples: 1998-2004 Intrepid/Concorde/300M; 1996-2000 Stratus/Breeze/Cirrus.
You may be able to take those jump-start terminals right from the junk car, or if not, you'll have an idea how to make your own.
--Have your friend take the time to learn how to open up his car using an automotive lock-out tool. This would be the most reliable method, as even if the car's electronics should fail, he'll still be able to get in.
Does his Integra have frameless windows? If so, opening the car will not be difficult.
--Your 9-volt battery idea is a cool one. Couple of things you might need to know, though, if that's how you plan to go about it.
If you're not aware of how power doorlocks (or relays in general) operate, take a look at the "Relays" section on this site.
Both wires going to the factory actuator "rest" at ground------when you operate the power lock switch, the relay takes away one of the grounds, and puts a positive there instead----that's how the locks work in pretty much every car.
So, if you simply jump a batery onto the two wires going to the actuator, you'll have nothing but a short circuit---and the car will not unlock.
What you would have to do, in this case, is completely disconnect the actuator from the car, and THEN you can apply a battery to the actuator.
So......... to do what you want, here's pretty much how you would do it. If you want to spend a few bucks on connectors, you could do a very neat job.
1. Find the two wires going to the driver's door actuator (actually it could be any one door; doesn't matter).....you can surely find them in the car; no need to go into the door.........anyway, cut them in half.
2. Extend BOTH sides of the wires you cut in half, using extensions long enough to reach outside the car.
(It would be wise to label BOTH ends of each wire, "car side" and "door side.")
Choose a location that the car's owner can get to without any tools, but is somewhat protected from road spray, and hidden from thieves.
3. At the auto parts store, look at all the different accessories for trailer wiring. You'll find a nicely made, weatherproof 2-pin connector. Buy a male/female pair........and also one more; doesn't matter if the third one is male or female.
4. Anyway, back outside the car, you now have two pairs of wires.......those are the ones you extended from inside the car..... one goes to the actuator, one is coming from the car.
Connect those two trailer plugs to the wires, so that you can connect the two pairs......so that the car is basically back to the way it was from the factory, except that you have that plug outside the car.
At this point, the car's power locks should operate normally.
5. Undo the connection, and get that third connector you bought-----it should fit into the "door side" of the plug that you made previously.
Splice that third connector onto a 9-volt battery connector, if you can find one at an electronics store.
Make a temporary connection at first, figure out which way you have to connect it so that the 9-volt UNLOCKS the door, and then make the connection permanent.
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So, all your friend should have to do is, find the hidden connection under the car, unplug it, and plug his 9-volt batter with the adaptor you made, into the "door side" of the plug.
Those trailer plugs are nice in that your friend will find it impossible to incorrectly plug into the "car side," because both plugs are different.
Obviously, if you do this, you will have to take special car about which wire is "car side" and "door side"..... don't get the polarities mixed up.....and make sure you put the correct plug on the correct wire, so your friend will be able to connect his battery to the "door side" of the wire.
If you do any of the above stuff wrong, the worst that'll really happen is you'll blow a fuse or that one doorlock will work backwards........but it's disappointing when you do a nice careful job making connections, and then you have to cut them apart and do it over. Much less stressful if you take a pen and paper and map it out the first time.
Oh, one more thing: First, make sure a 9-volt battery will actually power up the actuator! If that doesn't work, car-alarm-remote batteries are 12 volts; try one of those.