top dead center of the largest metal reflector surface usually has the best ground plane, but most of the mag mount ducks which are great for wheeling cause you can put them somewhere low profile get used a lot too simply because of the ease of their placement
but since you're in such close proximity to your fellow wheelers in most cases the higher SWR isn't a big deal, still should tune it to the radio, and check it in a few common and convienent locations so not to damage your radio though
my rubber duck antenna is about 1.6 SWR across the band at minimum wattage and it hits about 2.4 SWR at 12 watts which is like a 25% loss but I use the same mount location for both of my antennas, i just bought an extra antenna side WQD mount piece for the duck
I found that i raked alot of leaves with the 4 foot on the roof, only takes about minute to get out and swap them
you could stick a mag mount to the inside of your headliner and be able to talk the quarter mile or whatever to your wheeling buddies
like someone else has said, you could hook it up to a light bulb and put it in your floorboard and do the chatter
