took the plunge and punched the roof

[K3GM]

I have mine about 2.5' apart and see no interaction.  I use the coax that came with the Larsen NMO mount.  I believe it's RG-58/U.  I was able to leave the headliner in place, and access the roof through the dome lights.  I puuled the D pillar moulding off by the rear hatch and used a wire fish to get the coax from the mounts and down the pillar to the floor.  Like you, I also encountered the air bag cylinder, and just kept the coax near the wiring harness that passed by it.  My installation process is in K0GB's photo gallery if you'd like to see some photos.

[N6AJR]

I just finished up the last of my antennas on the truck. So now I have 4 in the roof and 4 on the bed rails.

 On the roof is a 900 mhz, an 2m/440, a 220 mhz, and a 1.2 ghz. and on the rails are a 2m, and a 2m/440 mhz, and on the other rail is a DK 3, and a ATAS 120.

 I have a stack of 5 radios in the cab, and some have multiple outputs. I have a dedicated MOTO 900 mhz FM radio, a dedicated 220 mhz FM rig, a dedicated 2m FM radio, an FT 857 with a choice of the ATAS or the DK3 for HF and 6 m, and a dedicated 2m/440 for the other side of it.  And I have a TM 742 with a dedicated  2m/440 antenna run through a  diplexer, and of course a dedicated 1.2 g  antenna for the 3rd band.

So lots of radios and lots of bands, but the truck was bought for a radio toy and is used for mobile use  for stuff like a Rover, or handing out grid squares, counties, and such.

 I even have a ladder rack on it to carry the odd large antenna if needed.

[KD8DEY]

One thing I've noticed about my new roof mount is how flimsy the sheet metal is up there. I put the mount in the flat spot between the stiffening ribs in the roof, giving it about 6" of flat, un-reinforced sheet metal between the ribs and nothing ahead or behind it. (it looks pretty good )

When I put a tall, stiff, and rather heavy antenna on the mount and moved it back and forth (as the wind might do while driving), I saw how it flexed the roof's sheet metal. This caused me to wonder what would happen if I accidentally drove into a garage or drive-thru with a stiff (no-spring) antenna. I'm guessing that it would be bad news for both the antenna and the roof.

For now, I have a stubby, 12" antenna up there, so I don't have to worry. But, I don't think I want to drive with anything longer that isn't spring loaded.

I have seen people use circular saw blades to reinforce the roof using SO-239 type mounts....