I made mine. Reason? That's pretty much what the factory did.....When I pulled my seats out of the donor Eldorado, the factory STOCK main wiring leads from the two bucket seats (12V, & Ground) were soldered together, then shrink-wrapped, leading into a factory connector, then to the circuit breaker/fuse box.
For our street rod application(s) 10 gauge 12V, & ground wiring is the size to use for the offered 30 amp load. Either use the proper sized butt-connectors, or solder the connections if you prefer, but make sure to SHRINK-WRAP the connections, regardless. For further protection, I would also split-loom the entire harness, as the seats are one of the biggest amp draws in any vehicle.
My seat wiring goes to one of these, or you could use something comparable (pic):