No wiring diagram or additional technical info became available last week, so I had to figure it out myself. The diagram in my previous post is incorrect for a European diesel-engined 626.
Behind the right headlamp on an easily accessible metal bracket, one can find the low-beam, high beam, and horn relays, in that order counting from the fender inwards. There's also a fourth relay on this bracket but I did not determine what it was for (might be for rear window defroster).
There are two 15A mini fuses, one for both low beams, and the other for both high beams, on the opposite side