Go to this site Club Touareg Forums
and do some searching. Some models came with the harness wrapped in foam and stuffed wayyyy up under the dash. My 2004 had this. On my 2007, I had to buy the module for the harness at the hitch but run two wires up to the dash to the brake controller and connect under the panel by the right tail light. Have no idea why VW left this out, but it was fairly easy to run under the back seat, carpet and sill plates.
Can not remeber the details but Club Touareg.com has the answer to your question. You just need to do some searching over there for your 2008. Someone has already done this and posted directions and photos. Then you can hook up whatever BC you want.