My antenna sits on my dash,beside my GPS antenna, where there is no metal anywhere nearby.
I really dont think that cable length is important,as the antenna that came with my receiver had about 4o feet of cable attatched to it, but a $2.oo japanese generic replacement from EBay came with about 8 feet of cable, and I cannot tell the difference between it, and the original, as far as its operation.
I also use a generic replacement antenna for my garvin GPS that I purchased from EBay for $4.00 (6.00 shipping) since my GPS cannot find satellites where its mounted. The GPS almost jumps off the dash when the antenna is plugged into it, the signal strength meter goes to its maximum bars.