Ambient Temp. Therm. Going Nuts!!

To all gurus of MB mystery.
My 1986 420SEL Ambient temperature therm. on the dash began reading in the 120s F about a week ago. Sometimes it starts in the lower 100s and just keeps working up in temp. to peak about 128. I checked behind the license plate (where the sensor is located)to see if was really that hot there but it wasn't even close. Do I need to replace the sensor or is there something else I can try first.

Thanks in advance,
Garrick
'85 300CD-T
'86 420SEL
'94 Land Rover Discovery