The w123 was never offered with power seats (at least not in the USA market) so there's not supposed to be any wiring or fuse slot for it. It's a custom installation of w126 seats into a w123. After messing with the wires on the seats and switches, all it boils down to is getting a power wire to each seat, and providing each seat with a good ground. The ground was easy.
If what I've read is right, then I'm looking at a possible 25A draw from each seat. I know that's not going to happen often, and probably not ever both seats at the same time, but I'd like to definitely give the seats their own 30A circuit to avoid maybe overloading some other circuit.
The only optional (powered) equipment my car does not have is the power sunroof (the w123 wagons had manual sunroofs). However, I checked, and the power sunroof fuse slot is not powered - there are no wires going to or from it.
I think what I'm going to do is run 4 new power wires from the battery, along the firewall, through one of the grommets on the driver side, to the area behind the fuse panel. 2 of them will be used for the seats, one each, run from here to the seats. I'll use an inline fuse holder near the seat, under the carpet. The other 2 wires will be hooked up to the fuse panel for future projects I have in mind (which draw less current
).
Thanks all for the ideas!