AM Radio gets static when driving near PV arrays, but not FM
It doesn't matter what you are driving next to. AM is more susceptible to noise. An AM transmitter is basically a filter with gain. If there is noise on that channel you are going to hear it. 

FM has a carrier frequency, when you tune your radio with FM the radio in it 'locks on' to that frequency an only information around the carrier gets sent to your speaker. This is not as susceptible to noise.

PV arrays are probably noisy because of DC to DC inverters and poor EMI control and which could also be harmonics of them converting to AC which would be lower frequencies around AM also. 

If you drive next to power lines you get more noise on AM when you are close to the poles (power lines don't have as much regulations as consumer devices as far as emissions go) but FM comes in clear.