Since you've got a '96 I assume your #15 fuse is 10 amps and for the following components:
Audio System, Courtesy Lights, Door Open Light, Trunk Light, Glove Box Light, Remote Control Central Locking System, Oil Level Sensor, Seat Belt Reminder, Data Link Connector.
Turn ignition to OFF, turn off all interior lighting (by operating the selector inside the car), close all doors, wait about 20 seconds and insert a new fuse. If the fuse is blown immediately upon insertion my guess would be that you have an aftermarket audio system with faulty wiring or maybe something is wrong with you Data Link Connector. If that is not the case the problem might prove a little bit harder to find, and you'll definitely need a wiring diagram. Check this site:http://www.volvoclub.org.uk/volvo_workshop-850techdocs.shtml
If the fuse is not blown immediately upon insertion: Good, the components affiliated with the fuse can then be grouped by what ignition position they are active at (OFF, 1, 2) and tested individually.
- Door open light, courtesy lights, glove box light, trunk light, remote central locking system.
Make sure the glove box light really is turned off when closing the glove box.
After that it's simply a matter of testing these components one after the other (with the ignition at position OFF) by opening/closing doors, turning on/off trunk light etc, checking the fuse between each component test.
Make sure that the interior lighting is turned off when you test the central locking, or else you might end up faulttracing central locking when the problem was with the interior lighting :)
(Since the interior lights can be set to turn on when the central locking is operated).
- Usually the audio system, unless it's an aftermarket system.
If the fuse breaks the second you turn the ignition to position 1 the audio system probably has faulty wiring.
If the fuse breaks the second you turn the ignition to position 2 one of these two components is the culprit:
- Seat belt reminder, oil level sensor.
Fasten all seat belts and try again with a new fuse.
If the fuse still breaks the oil level sensor is most likely the culprit.
If the fuse didn't break the seat belt reminder is the culprit. If so you can simply disconnect the seat belt reminder relay, ridding you of both your fuse problem and the annoying seat belt reminder. The relay is located behind the cover over the drivers feet (easily removed, just a couple of torx screws, no breakable interior clips).
The central locking relay and the alarm module are also found there, as well as some other electrical components.
The Data Link Connector is an unlikely culprit, but it's probably best if you unplugged it completely during testing.
If everything turns out ok with it disconnected you can test it at the end. (The Data Link Connector is the OBD II conncetor located by the coin holder, in front of the gearchange).