A few weeks ago, the O/D would not turn on. I had stopped to wash the car, got in and started and the light was on. Assumed I had hit the switch (poor placement by Volvo if you ask me). But it would not turn back on, so I ended up driving it like that, and it eventually came on during a stop that day. I did my Brickboard research, and it did not happen again until today.
I remembered the fuse, so at the next light, I reached down and removed the fuse cover and saw that it was fuse 11. As I got up to speed again, I counted down the fuses as I touched them, and turned #11. The light immediately went out, and I shifted into O/D.
Always check the simple stuff!
I'm not sure if this is related, but both times I had stopped while the rear defroster was on. Is this a relay to the rear defroster that could be the actual culprit?
Anyway, I'm not sure what other repairs you can do "on the fly", but it makes me think up some funny scenarios.