[Antennas] Proximity of Antennae On Top Of Car??

[DavidE Benedict]

Hi All,...

I am back again after a year or more absence.

I want to install a 6M vertical on my Subaru stationwagon roof.

I already have a (Comet?) 2M-440 combo VHF/UHF up there on the roof at the
center (side-to-side) and slightly forward of roof center (front-to-back),
and a Screwdriver antenna on the front (passenger side) bumper.

Surely there must be somebody with practical experience on this one... 

NOTE#1: I use an IC-706, and the 6M and the VHF/UHF feeds are at different
PL-259's on the back of the radio.

NOTE#2: The HF Screwdriver antenna and the 6M antenna are fed thru a coax
relay so that both are never connected to the IC-706's HF/6M RF connector
at the same time.

My questions are: 
1) How much trouble am I in with interaction of these three antennae?
2) When I am 6M I wonder if the Screwdriver and/or the VHF/UHF antenna
might act inadvertantly as a director element if my spacing is wrong...?
2) Are there exact rules, or rules of thumb about their juxtaposition?
3) If there are possible problems, what are they, and how do I minimize or
eliminate them and still have these two antennae up there on the roof
together?

TIA for any help...

W7DBH
David B.