OK, I'll start and stop at the first sentence or so.
The headlight and most if not all of the relays (there are two in the engine compartment, FI and interference, nuff said) are on the board behind the fuse panel in the center console. They're supposed to be able to slide out somewhat when you pull the ashtray and the fuse panel cover but they're often hard to move with all the wires there.

I've had to replace the fuse panel itself and it's generally easier to remove both sides of the center console.
You'll have to pull the equalizer if so equipped, the cig lighter, and the radio, but once you're in there everything is revealed.

I don't understand you couldn't find the headlight relay...it's the biggest round and orange thing on the relay panel.

Do you have a Bentley manual for your 90? At the level of detail you're getting into, you might also invest in the Volvo wiring manual/book.