Placement of a Mobile Antenna 
From Alf Enterprises web site at http://www.alfenterprises.com/AntennaIndexCategory.html

We have provided you some guidelines for mobile antenna selection. When selecting a mobile antenna, there are a number of factors that significantly affect the ultimate performance of the antenna. Gain requirements, electrical type, ground plane availability mounting style and placement, coaxial type and loss ratings, physical size, appearance, and surrounding environment are all issues that must be addressed to ensure the maximum performance from a mobile antenna installation. The electrical type or design of the mobile antenna is commonly referred to in terms of its dimensions in terms of wavelength: 1/4 wave, 1/2 wave, 5/8 wave, etc. Each electrical type has a specific radiating pattern to be considered when selecting a mobile antenna. For example, the signal radiating from a 1/4 wave antenna is directed more vertically, thus making it ideal in urban environments where buildings might obstruct the signal. The design of a 5dB collinear mobile antenna is designed to direct the signal more towards the horizon. This type of antenna is ideal for geographically flat regions where signal coverage is sparse.
Ground plane availability is another critical factor in mobile antenna performance, and must be considered when determining the location and type of the antenna. Ground plane requirements vary given the type of mobile antenna and the frequency of operation. A typical 5/8 wave antenna at 150 MHz requires a ground plane of at least 42” in diameter. At 450 MHz, 15” is required, and 800 MHz, a minimum of 8” is considered sufficient.
In terms of mounting mobile antennas on a vehicle, there are five general locations: the roof, front fender, rear fender, trunk and rear window glass (although other glass mount locations may be used). Of these, the center of an automobile roof is considered the best for mobile antenna placement, followed by the center of the trunk lid, the fenders, and then on-glass mounting. This ranking is determined by the amount of ground plane provided by the positioning, and clearance from obstruction (i.e.: the roof line), and is the reason the center of the roof is considered the ideal mounting location, provided the roof is metal. The diagram above illustrates the effective loss (at 800 MHz) due to insufficient symmetrical ground plane.