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 of the engine compartment near the strut tower and air box. Low beam fuse supplies power to the low-beam relay, and high beam fuse to the high beam relay. The DITD engine's air box is where the main fuse/relay box is located in models with other engines (and shown in the picture in my previous post).
Battery voltage should always be present at these two fuses (terminals closest to the fender), and is supplied by one white wire that goes through the wiring harness to a large grey multi-pin connector located under the air box. There's also a 40A maxi fuse for headlamps at the fuse block attached to the battery positive terminal, and power to that white wire eventually originates from this fuse.
The white wire connection in the large grey connector under the air box was responsible for the problem of inoperative high and low beam headlamps. I tried to clean the connectors, they were just too corroded be able to provide a reliable connection. The poor connection allowed a no-load voltage to the headlamp circuit of between 8.5V and 11V, but not enough current to trigger even one of the two relays.
The connector pins for this connection are small, resembling pins for a signal wire rather than a power wire that needs to carry up to 18A when both low and high beams are in use (2x H7 lows and 2x H1 highs at standard 55W each).
My solution was to cut the white wire before and after the grey connector, and solder in a new piece of wire to bypass the connector, leaving it long enough to still allow disconnection of the connector.