OK, so I found a page online that mentioned my symptoms as a subset of theirs. They also had TRACS light ON. This thread referred to a relay 106, and swapping with relay 105 to check to see if 106 was the cause of headlights and signals not working. This was also a 1998 S70. This appeared to be in a kidney shaped box in the engine compartment, containing both the long rectangular fuse area with 30 or so fuses as well as 8 relays labeled 101 through 108.

I found a somewhat similarly shaped box labeled "Main Fuses" near the strut tower, but that looks like about 10 fuseable links and 4 relays associated with the engine. Up near the windshield I have the 30 or so fuse block, but they are under a rectangular lid, not like the diagram I saw in the thread (https://www.matthewsvolvosite.com/forums/viewtopic.php?t=21088). No relays. So, I went to another possibility - the ignition switch...

EDIT 2 - I just saw a video where the cover of the 30 fuses is attached to a larger cover that conceals the relays - but the problem appears to be not be a relay after all...

I found that if I jostled the switch a bit with the car running, the headlights came on for a moment. Then I shut off the car and went to position II, and found the headlights and signals work fine with the car not running. Back to running, and no lights or signals. With some further jostling, the lights and signals work. It appears to be the switch.

EDIT 1 - I just saw a video of servicing the ignition switch, by removing the plastic shroud around the steering column. It looks like the contact can easily be accessed. I should be able to use some contact cleaner there. It looks deceptively easy.