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?
Sorry to not have more constructive suggestions.