Dave, K9KBM, has provided us with some useful insight regarding guidelines by automotive manufacturers, relating to the use of RF in vehicles.
If you have a newer car you might want to check your owner’s manual under the heading of “Electromagnetic Compatibility”.
Not only are there warnings to keep antenna and power cables at least 4″ from electronic modules and airbags, it limits power output to 50W on all frequencies except 806MHz-870Mhz which is limited to 10W. There are also images showing the placement of antennas at different frequencies.
The following examples are taken from Dave’s owner manual.
73, Mike K9KMV