Wanting to add an aftermarket Amp to my OEM stereo (SC816). Volvo dealer wants $160 for the pre-amp cable out - it also includes wiring harness, I don't think I need more than just that cable but dealer says it's sold as a package. Should I be able to just buy the pre-amp cable elsewhere, if so, where?

I've wired it before. $160 is a lot.

For about $12.99 and up you can get the pre-amp out piece made by Jensen's Road Gear at best buy or sears.
Splice the pre-amp device into your rear speaker wires (this is just so you can get the signal from the head unit), I would use some clamps to make it a cleaner install and it's more secure. I do not remember the name of the clamps at this moment but it involves two pieces, one part goes onto the speaker wire, the other part goes onto that piece and has the other connecting wire from the pre-amp output device. If it comes to me ill post it later on.
After that hook in the RCA cables into the other side, ride them to your amp either underneath the car or lossen some carpet on the side underneath the rear seat and bam, it's set for pre-amp.

If you have never done it before or don't know anyone that could assist you. I suggest going to a mom and pop audio store and they should wire your amp (power and pre-amp) for $50 or so. Does not take more than 45 minutes.