Did this a few months ago, and got about a 50% reduction in reception, and i live in town. I lost my oldies radio station from Las Cruces completely, which is my decent to marginal reception benchmark.
Was surprised it wasn't worse, since 1/4 wavelength antennas rely on being 90 degrees from ground plane (the body, ie VERTICAL) to be in-phase with the transmitting antenna's signal. Not only that but metal bodies are excellent shields for antennas. (One reason why you can disable Onstar with a coke can over the antenna).
When I did it, I could get ok reception when facing one way (crack towards the transmitters on the mountains), and poor to no reception turned 180 degrees.