I bought one on Ebay that comes with 3 different lengths. I settled on the mid size of about 6". The 2.5" had really poor reception but I'm out in the country. Whiterock is pretty urban so the 2.5 should be OK unless you're driving in the mountains.
i cant remember what size mine is but its the stubby one off of ptuning.com...looks awesome! reception = crap.. i usually listen to cd's anyways so I opted to deal with the crappy reception