Tan or "Brown" wires in all Volkswagens are ground wires. Depending on the length of this wire and if it has been tampered with or not, it should attach to a white connector behind the dash, just to the left of where the fan actually sits. This connecter may be attached on the backside of the dash itself. If it is longer it may have been wired to the common ground star prongs under the steering column. Hope this helps.