Thanks, guys.
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I already removed the glove box, but all that does is allow me to see the relay in question, not gain access to it, unfortunately.
This was never an issue before we started having kids. I don't mind opening my door and then reaching across to open the passenger door, but when carrying at least one kid, my backpack, and the diaper bag, it's a huge pain to juggle kids around to unlock my door, then reach in to unlock the back door, and then repeat on the other side of the car. It wouldn't be so bad if they had just installed key locks on the back doors, too, but I guess since it came with power locks they didn't think it was necessary. Sort of like how a kick-start wasn't necessary for the 650.
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After looking around some more I'm finding a lot of aftermarket keyless systems that say they come with built-in power lock relays, so I'm hoping that's what I'm looking for. I'm gonna try to get to a store today to look at some in person.
The only problem after that is that the driver's door doesn't actually have a power lock in it. Since it didn't come with a factory keyless system, the driver's door is unlocked manually which throws the switch to unlock and lock the other doors, so I'll have to either install an actuator in that door, or just stick with unlocking that one with the key, which is an acceptable compromise.