More info from my Tech Head brother:

"If as one of the other messages indicates the roof rack is not grounded you end up forming an unbalanced dipole when the conduit is in electrical contact with the roof rack. This is a "bad thing."

When you isolate the conduit with electrical tape, the antenna mounted in the middle causes it to from an approximately quarter wave ground plane. RF ground is provided through the shield of the coax and balanced by the conduit. This gives you much better SWR and is perfectly OK.

Sounds like you've got a solution. Just bear in mind that you do not want an SWR above about 1.5 if you can help it. Also you want your lowest SWR at channel 20 and channels 1 and 40 should be approximately equal SWR. If 40 is higher shorten the antenna, if 1 is higher lengthen it."

I'll let you all know when it works.