On our older Volvos, the same fuse (No.8) applies to the central locks, interior lights, clock, etc. If the dome light works, the fuse should be OK, but watch it as you try the locks...if it dims or goes out when you operate the locks, there is a bad connction - probably at the fuse ends (corrosion likely). If the fuse is OK, try the below.

All the 240's I've seen have a connector behind the felt padding above the driver's legs and just left of the steering column. This connects the door harness to the lock/unlock relays. Wires have always been Green-Black-Red as I recall. Drop the padding, find this connector and unplug the halves.

Using the half that contains wires disappearing toward the centre console area, and using something like a paper clip, short the Black to the Red, then Black to Green. Don't worry about sparks - you are only using a fraction of an Amp to trigger the relays. One of these actions should cause the doors to lock, the other to unlock.

If this works, your problem is in the driver's door wiring and/or switches, but, as both functions are dead, I would suspect the Black ground wire. If it doesn't work, you have a problem with the relays or their power supply. This would be uncommon.
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Bob (son's 81-244GL B21F-M46, dtr's 94-940 B230FD, my 83-244DL B23F-M46, 89-745(LT1 V8), 98-S90, 77MGB and four old motorcycles)