There are dual band antennas that would fit on your roof and into your garage without interference, if you're willing to give up the couple dB of gain of your longer Diamond antenna. In practical terms, you wouldn't tell the difference. There are other antenna technologies like the DRRR that are low profile you can mount on the roof as well, but aren't dual-band. But with a diplexer you could install an antenna for each band and still enjoy simultaneous V/U operation. I once knew a ham that gamma fed the luggage rack on top, there's always antenna options.
If there isn't a hole in the firewall for wires, make one. Add a grommet and seal it with silicone when you're done.
How fond are you of those fiberglass bed covers? One of those with some sheet aluminum contact cemented under it would be a great ground plane and allow a slightly longer antenna. A pickup bed toolbox would afford similar options.