Since you got as far as the soldered board, look carefully at the (groups of six) round pins that protrude through from each switch. I have had to resolder these connections, driver and passenger seats, on EVERY one of my 700 cars for intermittant operation...and it has cured the problem 100% every time.

There is a separate 25A fuse for each seat. If you pull the appropriate fuse and work with the ignition off, you can resolder the board (requires desoldering most of the wires to get the white insulator off) while it's connected to the seat.