You might consider getting a wiring harness from the dealer. More costly, but probably easy to install and you won't have problems with the bulb-out indicator lighting up.

I need to disagree with the comment about the 850 being a poor choice for towing. An auxiliary ATF cooler is always a good idea (along with regular ATF flushes), and you should avoid towing any significant load in 4th (frequent shifting back to third is the worst thing that you can do). It seems to be the heat, not the load that causes problems. However, towing is not an afterthought for Volvo. I've had more than 3,000 lbs behind my S60 and it handled it very well. A thousand pound trailer that is well-balanced is hardly noticeable. The S60 has independent rear suspension and a wider rear track, but I would expect the 850 to do fine as well if a little thought is put into hitch setup and the load being towed.

Check out http://www.theautochannel.com/news/2006/02/27/211961.html