Hello, and Thank You for using Just Answer.
This is a 2 part question. For the sometimes fast turn signals: It is common to see failed front signal bulb and socket assemblies. They can lose contact in the bulb socket base "intermittently", and cause what you are describing. If you want to catch the failed one rather than replace both, try this: When the signals are flashing fast, turn on the hazard switch( 4 ways). Look carefully at the front signals. With the key on, meaning running lights on, both sides may flash, but one may go completely out during the flash cycle. There are two filaments in the bulb. If it goes completely out, the signal is countering the park filament, and is the failed component. It still might be worth your while to replace both front socket and bulb assemblies (They come in a package together at the dealer). Note, the RF side is opposite of the left side in terms of removal.. It is removed by turning to the right, and installed to the left. NOW, about the position lamps!! The display in the cluster will show: Bulb Failure Position Lamp for the 4 tail lamps. Bulb Failure Dipped Beam for headlamp bulbs. Bulb Failure Multiple Bulbs usually indicates tail lights, brake lamp, and headlamp failure. Make sure both low beam bulbs are working, and then we can concentrate on the tail lights and brake bulbs. The bulb failure circuitry is able to detect small changes in "expected" current flow. You need to make sure that the correct wattage bulb is installed in each bulb socket. The exact bulb rating escapes me now ( 5W I think), but the Volvo tail lamp bulb should have a red base. The bulb specifics are stamped right on to the bulb base. Have the parts personnel look up and verify that all 4 bulbs are correct. If all bulbs are confirmed to be correct, and are in fact working, including brake bulbs, the REM module (Rear Electronic Module) would be next to look at. Also, I've seen "shunts" and terminal issues on the CEM(Central Electrical Module) cause headlight bulb failures. Also, a shunt on the REM could have failed, causing the bulb failure to come on, even with correct bulbs. Shunts are rectangular/flat components plugged into the REM and CEM. They are the "comparerer" so to speake. In other words, shunts are what determine if a bulb is failed or not. I have NEVER had to replace a harness on an S60 for lights though. The REM and CEM would require a tech to fully diag and replace.
Let me know how you make out. Can't wait to hear!!!!!! Bill