Mark, all bulbs are working fine. I know what you mean though, about overlooking the obvious!! Been there before.
I forgot to mention that I have not wired up the fog lamp wiring since my trailer does not have a fog lamp. I have left this bit of wiring intact in the original socket. The lamp did work (on car) the other day when I tested it with trailer connected. This is not monitored though is it?
I don't know much about the Saab trailer socket arrangement, but I do know that you have to be careful when wiring trailers to car with bulb failure systems so as not too upset them. I believe that I've read on here somewhere that there's a socket already in the loom that you can plug the trailer wiring straight into. The only experience I've had was with a Mercedes where we had to use a lot of relays to supply the trailer power, using the existing loom only for switching signals so as not to upset the bulb monitoring.
You do realise that you have two rear light bulbs on each side, so if one fails you still have light on each side. It can be easy to overlook the failed one, especially if it is the inner one. My warning came one last night as I left for home. It was the inner right rear light, but as I still had light at each side I decided to drive home and change the bulb, rather than do it at the road side. Part way home it started working again. Weird eh?
If you are using the spare trailer socket in the boot this is after the bulb monitoring so extra current to trailer should have no effect, maybe you disturbed something or a bulb has failed, note the bulb monitoring is sensitive enough to show a warning from 2 working bulbs drawing slightly diff ampages so look for corrosion on connections, sockets, wiring etc.
I have a bulb out!! Is that a coincidence with fitting the towbar or what!! Hope it does not blow a new one. I never realised that there are two bulbs on each side.
If anyone is interested, I put a towsure towbar on.
It took just 5 minutes to bolt it all together, but another hour to wire up and check all. No need to remove bumper as indicated in instructions.